Tried this, as my first ever dual boxing... (pretty much) alt+tab or in my case two monitors, can work but issue I always had was that, to hold agro in an instance I needed to drop fury and go defensive, but then I don't have the damage to kill. If I stick to fury (arms/berserker) then while I can hold the agro on two or three, any thing else slips past and kills the priest (in my case druid). And this was stockades so nothing especially high level.

Main issue is though if you're doing your damnedest to stay alive on the warrior, you don't have the depth of awareness to keep an eye on the priest to make sure it doesn't have a stray add beating on it. By the time you notice it has an issue it will be too late for you to alt+tab back heal priest, and pull the add back into the group you're playing with.

IMO