To put the images from two cameras on one full screen, you'll either have to stretch the two images. If you want to maintain the same aspect ratio, you'll only use only half of the monitor screen. (1)
So, even if there are only two cameras, using a quad camera system would still be a good choice. And you're ready to add more cameras in the future.
Note (1): You could rotate the monitor and use more of it's screen. The way to do that would be running a PC with two video capture cards, and using software to rotate and scale the two inputs.