I usually use two keyboards for my grouping. With the second keyboard on the right controlling all my secondary chars. (ie; Priest, Mage, Warlock, Druid, Hunter). Its really quite easy on that secondary keyboard to set up your key bindings in a logical and easy to find pattern.

The key I use the most actually is tilda ( ~ ). On every single secondary char this is bound to move backwards. So the way most fights flow is. Warrior charges and does warrior things on primary. On secondary keyboard I hit ~ stopping them in place, and start my sequence of attacks with 1,2,1 etc...

I'm not really sure how this would work on a computer running two wow programs. Does one keypress send info from the both clients to the server? Or do you have to alt tab? I don't know.

Just food for thought. I think the holy trinity would probably be the easiest.