This really depends on what boxes you are using.
Traditionally RS/6000 has an issue with KVM switches. The one thing I can tell you, is they have to be POWERED. As long as both boxes keep the connection alive, it should work okay. The only company I know that states they support RS is blackbox.
However, you could run into a problem that I have now. The new RS6000s, p5s, have switched to USB. There is only one known keyboard and mouse that work with it, and I can't get that to flow through a KVM>