user25116: I am confused how to do this thing with no-crop in android with ffmpeg,

Video details
video 1 
size - 700*400(width*height)
duration - 5 second
video 2 
size - 400*600(width*height)
duration - 5 second

Output video detail
video size - 700*600 (width*height)
Duration : 10second
I need this type of output please help me how to do this type of output with ffmpeg, graphical design is in bottom picture.
Tell me FFMPEG command .

after @gyan's answer i got this Error 
 onFailure: ffmpeg version n4.0-39-gda39990 Copyright (c) 2000-2018 the FFmpeg developers
      built with gcc 4.9.x (GCC) 20150123 (prerelease)
      configuration: --target-os=linux --cross-prefix=/root/bravobit/ffmpeg-android/toolchain-android/bin/arm-linux-androideabi- --arch=arm --cpu=cortex-a8 --enable-runtime-cpudetect --sysroot=/root/bravobit/ffmpeg-android/toolchain-android/sysroot --enable-pic --enable-libx264 --enable-ffprobe --enable-libopus --enable-libvorbis --enable-libfdk-aac --enable-libfreetype --enable-libfribidi --enable-libmp3lame --enable-fontconfig --enable-libvpx --enable-libass --enable-yasm --enable-pthreads --disable-debug --enable-version3 --enable-hardcoded-tables --disable-ffplay --disable-linux-perf --disable-doc --disable-shared --enable-static --enable-runtime-cpudetect --enable-nonfree --enable-network --enable-avresample --enable-avformat --enable-avcodec --enable-indev=lavfi --enable-hwaccels --enable-ffmpeg --enable-zlib --enable-gpl --enable-small --enable-nonfree --pkg-config=pkg-config --pkg-config-flags=--static --prefix=/root/bravobit/ffmpeg-android/build/armeabi-v7a --extra-cflags='-I/root/bravobit/ffmpeg-android/toolchain-android/include -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2 -fno-strict-overflow -fstack-protector-all' --extra-ldflags='-L/root/bravobit/ffmpeg-android/toolchain-android/lib -Wl,-z,relro -Wl,-z,now -pie' --extra-cxxflags=
      libavutil      56. 14.100 / 56. 14.100
      libavcodec     58. 18.100 / 58. 18.100
      libavformat    58. 12.100 / 58. 12.100
      libavdevice    58.  3.100 / 58.  3.100
      libavfilter     7. 16.100 /  7. 16.100
      libavresample   4.  0.  0 /  4.  0.  0
      libswscale      5.  1.100 /  5.  1.100
      libswresample   3.  1.100 /  3.  1.100
      libpostproc    55.  1.100 / 55.  1.100
    Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '/storage/emulated/0/Download/Received/VID-20190404-WA0010.mp4':
      Metadata:
        major_brand     : mp42
        minor_version   : 1
        compatible_brands: mp41mp42isom
        creation_time   : 2018-11-24T20:20:48.000000Z
      Duration: 00:00:59.51, start: 0.066667, bitrate: 649 kb/s
        Stream #0:0(und): Video: h264 (avc1 / 0x31637661), yuv420p(tv, bt709), 640x640, 646 kb/s, 29.98 fps, 30 tbr, 15360 tbn, 60 tbc (default)
        Metadata:
          creation_time   : 2018-11-24T20:20:48.000000Z
          handler_name    : Core Media Video
    Input #1, mov,mp4,m4a,3gp,3g2,mj2, from '/storage/emulated/0/Download/Received/Video_20171219203619535_by_vimady.mp4':
      Metadata:
        major_brand     : isom
        minor_version   : 512
        compatible_brands: isomiso2avc1mp41
        encoder         : Lavf55.19.104
      Duration: 00:00:44.54, start: 0.000000, bitrate: 17132 kb/s
        Stream #1:0(und): Audio: aac (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 128 kb/s (default)
        Metadata:
          handler_name    : SoundHandler
        Stream #1:1(eng): Video: h264 (avc1 / 0x31637661), yuvj420p(pc, smpte170m), 1920x1080, 17001 kb/s, 26.18 fps, 24.83 tbr, 90k tbn, 180k tbc (default)
        Metadata:
          rotate          : 90
          handler_name    : VideoHandler
        Side data:
          displaymatrix: rotation of -90.00 degrees
    Input #2, lavfi, from 'color':
      Duration: N/A, start: 0.000000, bitrate: N/A
        Stream #2:0: Video: rawvideo (I420 / 0x30323449), yuv420p, 320x240 [SAR 1:1 DAR 4:3], 25 tbr, 25 tbn, 25 tbc
    Stream mapping:
      Stream #0:0 (h264) -> scale2ref:ref (graph 0)
      Stream #1:1 (h264) -> scale2ref:ref (graph 0)
      Stream #2:0 (rawvideo) -> scale2ref:default (graph 0)
      setsar (graph 0) -> Stream #0:0 (libx264)
      Stream #1:0 -> #0:1 (aac (native) -> aac (native))
    Press [q] to stop, [?] for help
    [swscaler @ 0xb11b0000] deprecated pixel format used, make sure you did set range correctly
    [libx264 @ 0xb543b100] using SAR=1/1
    [libx264 @ 0xb543b100] using cpu capabilities: ARMv6 NEON
    [libx264 @ 0xb543b100] profile High, level 4.0
    [libx264 @ 0xb543b100] 264 - core 152 r2851M ba24899 - H.264/MPEG-4 AVC codec - Copyleft 2003-2017 - http://www.videolan.org/x264.html - options: cabac=1 ref=3 deblock=1:0:0 analyse=0x3:0x113 me=hex subme=7 psy=1 psy_rd=1.00:0.00 mixed_ref=1 me_ran


user1871: You need to use a canvas which can accommodate all inputs. To adapt this canvas during runtime, we can use the scale2ref filter.
ffmpeg -i video1 -i video2 -f lavfi -i color -filter_complex "[2][0]scale2ref[canvas][vid1];[canvas][1]scale2ref='max(iw,main_w)':'max(ih,main_h)'[canvas][vid2];[canvas]split=2[canvas1][canvas2];[canvas1][vid1]overlay=x='(W-w)/2':y='(H-h)/2':shortest=1[vid1];[canvas2][vid2]overlay=x='(W-w)/2':y='(H-h)/2':shortest=1[vid2];[vid1][vid2]concat=n=2:v=1:a=0,setsar=1" out.mp4
Audio is ignored.

To fit within a fixed size, use
ffmpeg -i video1 -i video2 -f lavfi -i color=s=854x480:r=30 -filter_complex "[0]scale=854x480:force_original_aspect_ratio=decrease[vid1];[1]scale=854x480:force_original_aspect_ratio=decrease[vid2];[2][vid1]overlay=x='(W-w)/2':y='(H-h)/2':shortest=1[vid1];[2][vid2]overlay=x='(W-w)/2':y='(H-h)/2':shortest=1[vid2];[vid1][vid2]concat=n=2:v=1:a=0,setsar=1" out.mp4

25116: <p>I am confused how to do this thing with no-crop in android with ffmpeg,</p>

<blockquote>
  <h3>Video details</h3>
  
  <p><strong>video 1</strong> </p>
  
  <p>size - 700*400(width*height)</p>
  
  <p>duration - 5 second</p>
  
  <p><strong>video 2</strong> </p>
  
  <p>size - 400*600(width*height)</p>
  
  <p>duration - 5 second</p>
</blockquote>

<p><strong>Output video detail</strong></p>

<p>video size - 700*600 (width*height)</p>

<p>Duration : 10second</p>

<p>I need this type of output please help me how to do this type of output with ffmpeg, graphical design is in bottom picture.</p>

<p>Tell me FFMPEG command .</p>

<p><a href="https://i.stack.imgur.com/2o0aY.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/2o0aY.jpg" alt="enter image description here"></a></p>

<p>after @gyan's answer i got this Error </p>

<pre><code> onFailure: ffmpeg version n4.0-39-gda39990 Copyright (c) 2000-2018 the FFmpeg developers
      built with gcc 4.9.x (GCC) 20150123 (prerelease)
      configuration: --target-os=linux --cross-prefix=/root/bravobit/ffmpeg-android/toolchain-android/bin/arm-linux-androideabi- --arch=arm --cpu=cortex-a8 --enable-runtime-cpudetect --sysroot=/root/bravobit/ffmpeg-android/toolchain-android/sysroot --enable-pic --enable-libx264 --enable-ffprobe --enable-libopus --enable-libvorbis --enable-libfdk-aac --enable-libfreetype --enable-libfribidi --enable-libmp3lame --enable-fontconfig --enable-libvpx --enable-libass --enable-yasm --enable-pthreads --disable-debug --enable-version3 --enable-hardcoded-tables --disable-ffplay --disable-linux-perf --disable-doc --disable-shared --enable-static --enable-runtime-cpudetect --enable-nonfree --enable-network --enable-avresample --enable-avformat --enable-avcodec --enable-indev=lavfi --enable-hwaccels --enable-ffmpeg --enable-zlib --enable-gpl --enable-small --enable-nonfree --pkg-config=pkg-config --pkg-config-flags=--static --prefix=/root/bravobit/ffmpeg-android/build/armeabi-v7a --extra-cflags='-I/root/bravobit/ffmpeg-android/toolchain-android/include -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2 -fno-strict-overflow -fstack-protector-all' --extra-ldflags='-L/root/bravobit/ffmpeg-android/toolchain-android/lib -Wl,-z,relro -Wl,-z,now -pie' --extra-cxxflags=
      libavutil      56. 14.100 / 56. 14.100
      libavcodec     58. 18.100 / 58. 18.100
      libavformat    58. 12.100 / 58. 12.100
      libavdevice    58.  3.100 / 58.  3.100
      libavfilter     7. 16.100 /  7. 16.100
      libavresample   4.  0.  0 /  4.  0.  0
      libswscale      5.  1.100 /  5.  1.100
      libswresample   3.  1.100 /  3.  1.100
      libpostproc    55.  1.100 / 55.  1.100
    Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '/storage/emulated/0/Download/Received/VID-20190404-WA0010.mp4':
      Metadata:
        major_brand     : mp42
        minor_version   : 1
        compatible_brands: mp41mp42isom
        creation_time   : 2018-11-24T20:20:48.000000Z
      Duration: 00:00:59.51, start: 0.066667, bitrate: 649 kb/s
        Stream #0:0(und): Video: h264 (avc1 / 0x31637661), yuv420p(tv, bt709), 640x640, 646 kb/s, 29.98 fps, 30 tbr, 15360 tbn, 60 tbc (default)
        Metadata:
          creation_time   : 2018-11-24T20:20:48.000000Z
          handler_name    : Core Media Video
    Input #1, mov,mp4,m4a,3gp,3g2,mj2, from '/storage/emulated/0/Download/Received/Video_20171219203619535_by_vimady.mp4':
      Metadata:
        major_brand     : isom
        minor_version   : 512
        compatible_brands: isomiso2avc1mp41
        encoder         : Lavf55.19.104
      Duration: 00:00:44.54, start: 0.000000, bitrate: 17132 kb/s
        Stream #1:0(und): Audio: aac (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 128 kb/s (default)
        Metadata:
          handler_name    : SoundHandler
        Stream #1:1(eng): Video: h264 (avc1 / 0x31637661), yuvj420p(pc, smpte170m), 1920x1080, 17001 kb/s, 26.18 fps, 24.83 tbr, 90k tbn, 180k tbc (default)
        Metadata:
          rotate          : 90
          handler_name    : VideoHandler
        Side data:
          displaymatrix: rotation of -90.00 degrees
    Input #2, lavfi, from 'color':
      Duration: N/A, start: 0.000000, bitrate: N/A
        Stream #2:0: Video: rawvideo (I420 / 0x30323449), yuv420p, 320x240 [SAR 1:1 DAR 4:3], 25 tbr, 25 tbn, 25 tbc
    Stream mapping:
      Stream #0:0 (h264) -&gt; scale2ref:ref (graph 0)
      Stream #1:1 (h264) -&gt; scale2ref:ref (graph 0)
      Stream #2:0 (rawvideo) -&gt; scale2ref:default (graph 0)
      setsar (graph 0) -&gt; Stream #0:0 (libx264)
      Stream #1:0 -&gt; #0:1 (aac (native) -&gt; aac (native))
    Press [q] to stop, [?] for help
    [swscaler @ 0xb11b0000] deprecated pixel format used, make sure you did set range correctly
    [libx264 @ 0xb543b100] using SAR=1/1
    [libx264 @ 0xb543b100] using cpu capabilities: ARMv6 NEON
    [libx264 @ 0xb543b100] profile High, level 4.0
    [libx264 @ 0xb543b100] 264 - core 152 r2851M ba24899 - H.264/MPEG-4 AVC codec - Copyleft 2003-2017 - http://www.videolan.org/x264.html - options: cabac=1 ref=3 deblock=1:0:0 analyse=0x3:0x113 me=hex subme=7 psy=1 psy_rd=1.00:0.00 mixed_ref=1 me_ran
</code></pre>
 1871: <p>You need to use a canvas which can accommodate all inputs. To adapt this canvas during runtime, we can use the scale2ref filter.</p>

<p><code>ffmpeg -i video1 -i video2 -f lavfi -i color -filter_complex "[2][0]scale2ref[canvas][vid1];[canvas][1]scale2ref='max(iw,main_w)':'max(ih,main_h)'[canvas][vid2];[canvas]split=2[canvas1][canvas2];[canvas1][vid1]overlay=x='(W-w)/2':y='(H-h)/2':shortest=1[vid1];[canvas2][vid2]overlay=x='(W-w)/2':y='(H-h)/2':shortest=1[vid2];[vid1][vid2]concat=n=2:v=1:a=0,setsar=1" out.mp4</code></p>

<p>Audio is ignored.</p>

<hr>

<p>To fit within a fixed size, use</p>

<p><code>ffmpeg -i video1 -i video2 -f lavfi -i color=s=854x480:r=30 -filter_complex "[0]scale=854x480:force_original_aspect_ratio=decrease[vid1];[1]scale=854x480:force_original_aspect_ratio=decrease[vid2];[2][vid1]overlay=x='(W-w)/2':y='(H-h)/2':shortest=1[vid1];[2][vid2]overlay=x='(W-w)/2':y='(H-h)/2':shortest=1[vid2];[vid1][vid2]concat=n=2:v=1:a=0,setsar=1" out.mp4</code></p>
