I would use diodes to isolate both the power and the signal lines. Also, just splicing the cord into a Y-cable would mean that whatever you did on one computer would happen on the other.
1) You want to be able to do the same exact thing on both computers at the same time. Which would require you to fine tune the sensitivity on both computers so they dont get off track.
or
2) You want to be able to switch back and forth and control only one computer at a time. With the Y-cable, while doing one thing on one computer, you would be clicking random stuff on the other.
What I would suggest is you use switches. If you can find an 8-10 pole double throw switch, you could just switch back and forth, just as if you were unplugging it from one computer and plugging into the other computer.
~Mike