ah, I misunderstood your problem.
get a KVM switch (probably an old version which does VGA and PS/2 switching through actually switching wires is better. This is what I use. I'd have to test the usb one (which I also have) to see if it would work as well).
also passthrough a USB something to the W7 in which you plunge the keyboard and mouse cables from the KVM (get KM ps2 to usb cable if needed). The VGA/DVI from kvm goes into your passedthrough ATI
same set of cables will go to the W8 machine
then actual monitor and mouse and keyboard goes into the kvm.
I have this setup, works like a charm.