There is a function when you are moving pieces around if you right click it will bring up a menu. The third option down will be to join/separate. Once you have selected that you just click on the line where you want to create the separation. I had to do this myself when I was pepping the thruster pack thingies on my Mk VII armor. Hope this helps!
I will likely need to find a printer that prints on larger sheets, but I'll check and see if there are seems where I can split them first -thanks for the tip .