If I am understanding you correctly, and you really need 2 USB inputs to 2 USB outputs, then you need what I've seen referred to as a "matrix" or a "matrix switch."
I use an HDMI matrix splitter in my cab -- see my sig link-- to send either my PC or my PS3 to
either, or both my living room set and my cabinet's monitor. It's a 2x4 matrix/splitter (2 inputs, 4 possible outputs.)
Just doing a couple of seconds of Googling, a full-on USB/HDMI KVM matrix seems to be prohibitively expensive (unless you have DEEP pockets.) That said, I did find this:
http://www.pccables.com/30080.html You could probably do better than this (or at least find a better vendor...)
What I guess I'm not understanding is why you couldn't plug both PS360s into a plain old USB hub, and then split the outputs with something like this (or with what BadMouth suggested?):
http://www.amazon.com/Kensington-ShareCentral-Device-Sharing-K33900US/dp/B0014IUVQA/ It is USB, after all, but again I don't think I'm quite following you... ?

Hope that helps...
EDIT: Wait, are you keeping the iPac, or removing it from the system? OP says "swapping out." I'm a little confused?