Well, I received my new Dell XPS 15 9510 (actually my daughter's laptop) and have it completely configured.  I added the second hard drive which was very easy.  I was surprised that the existing hard drive was in slot 2.  I installed my 2 TB hard drive in slot 1 and everything worked out fine. I have her old laptop and the new one siting side by side with the same background picture on both laptops.  It is amazing how much better the new one looks! I love the laptop!!!! I would like to thank everyone who commented on this subject.  They were all helpful much appreciated.  Thanks!!!
I have purchased a Dell XPS 15 9510.  I ordered it with a 2 TB M2 SSD Hard Drive.  I wanted another 2 TB M2 SSD Hard Drive in Slot 2 but that was not an option.  Since I couldn't  have the laptop built with the second hard drive, I ordered a 2 TB N2 SSD Hard Drive from Dell and plan on installing it myself.   Any thoughts or suggestions???   Thanks in advance!    
Installing a second hard drive