Best option that cannot possibly get you banned from WoW: 2 wireless keyboards. You hook up the recievers to both computers, but "synch" them to only 1 keyboard. Put all of you primary characters' abilities in their normal positions (keys 1234567890-=) and bind your secondary characters' abilities to the keypad (macros for healing your main, macros for assist nuking your mains target)

I use the Logitech Wireless Desktop 650 from Newegg.com, its $23 per keyboard.

*Note that you need to have the same brand/model for this to work.