I ran a warrior and priest up till lvl 70 on one pc, i just recently got keyclone about a month ago which would have made it alot easier... The best way to do this is either put your bars with the warrior set up and have the priest use a modifier (control+1 etc..) to heal with or use the PiP in keyclone or even just use half and half with your bar setup, i use it that way on my lock and priest for 2v2, but for instances you might want to get a different class setup as they both require micro managment...

Just my opinion though, alt tabbing sucked for leveling and i couldn't do it for instances at all i had to do one or the other.