I dont know how helpful this is for any more than 2 boxing, but I made a getto rig that helps me swap mains in the event of one toon kicking over.

I have a HUGE aversion to using the shift key in any of my keybindings, I will use CTRL+ALT before the shift... its just not something that I can personally use. I set my secondary toon to use the bindings as if it was my main and then set all of the actions/spells/macros to be SHIFT+the key that I needed. I also did the same for my main, so I can easily swap back and forth with basically a "shift key means I'm the secondary toon" sort of setup. I then push down the shift key on my second keyboard and jam a quarter in there to hold the key down.

If I need to swap mains for some reason I just move my quarter from one keyboard to the other and in event of an emergency I will flick the quarter out and take over with the loss of about a half a second (ok once it was a freak accident and when I flicked the quarter it actually just rolled up and held down the enter key and I sent a wall of gibberish to my guild. but that was only once)

It is a ghetto setup but I use it when I 2 box with wow now. I got the idea from a different mmo where if you held down CTRL you could spam a skill with the mousebutton, and shift did nothing so you could roll the quarter up to the shift key when you didn't need the ctrl pressed.