The P3421W does not have a DP out port, so attaching a second monitor to it is not possible. The additional 27" monitor would have to be connected directly to one of the laptops video out port. So that one laptop would be sending video signal to both the P3421W and the 27" monitor.
I would like to add another monitor to my set up, but I don't know how to do it. Can you help me? Currently I am using 2 laptops with a P3421W monitor. One laptop is connected to the monitor via USB-C and the other one is connected via HDMI + USB-A cables. The mouse and the keyboard is connected to the monitor. I frequently use the kvm switch functionality of the monitor to switch between the two laptops (I do not use pib, pip).  I would like to add another monitor to this set up, a 27 inch monitor which i would use in portrait mode. How can I do this? The only constraint is that I would like to keep the using the KVM functionality. I have 1 free USB A and 1 free DisplayPort port in the back of the monitor. If it is possible to do this, which monitor do you reccommend for this purpose?Thanks for your answers!  
P3421W, 2 monitors, 2 computers