IS there a way with multiplicity or any other software to have the KVM switch to show multiple computers screens on one monitor.
Not specifically but, for KVM connections, they can be Windowed. In other words, you could have any number of KVM (Pro) windows open and tile them on any monitor anyway you like.
Example I am on main computer and hit the letter g, where I program it to hit the number 1 on the second computer?
You can have it print the same key on the Secondary with this option, but not mapped to a different key: