i don't know if it's possible to split up usb but you can do this with midi.. check google/ebay for midi through boxes or midi splitter. this tiny little gadgets have one midi in and depending one how much money you want to spent more midi outs/throughs..
also you need to get a midi interface for your other computer..just one of this little boxes which provides the DIN5/midi plugs..m-audio midisport seems to be enough for this job.. then you connect your midi keyboard via usb to computer1 sending data on channel1? rewire the midi out from the master controller to midi interface connected to computer2
finally you can remap the midi channels with midi patchbay
depending on which channels are used as defaults by your midi controller..
debugging this things are a lot easier with apple's midi configuration tool, i think it should be somewhere in the utilities..