I've approached it a little different, i dualbox with 2 mages.
Normal 1 till + is to cast spells for both chars. on 2 fe is fireball for the one, and frost bolt for the other. So pressing 2 results in 1 frostbolt and 1 fireball.
CTRL normal 1 till ctrl0 however, only the main casts.
Numpad 1-0 only the 2nd toon cast spells.
I'm not happy with the CTRL solution though, maybe change it to ctrl+key is casting both.
I walk with wsad for the main and arrowkeys for 2nd toon.
This way they are pretty able to live a own life
With more toons I doubt this wil work haha.
I think with different classes this will work.
Will try it one day... I hope