What you need here is a Y-mouse ($60 from PI Engineering). It's also possible that the Belkin OmniView does the same thing for about half the cost ($33 from Amazon with free shipping, or $31 from Provantage). Xzin's hardware write up has an "ASCII schematic" that should be enough to figure out the wiring. It sounds like you have everything setup correctly, but that you are missing the y-mouse to allow multiple simultaneous PS/2 keyboard input.