before i added my other 3 accounts i duo'd a mage/priest combo for a couple years. I tried a number of different combos, for for 2boxing that was the one that stuck with me. Bringing a holy/disc heal specced priest along seemed to make PuGs much easier, and the mage allowed me to keep my sanity since the priest couldn't dps anything down to save her life. I think i am sitting at about 112 days played on those two.

aside:: Pocket priest for pvp is always fun too!

Prior to going the caster route, I duo'd a rogue/priest pair that was a lot of fun too (again, bringing dps/heals to the group helped make instances less painful, i would swap the priest for a druid in that setup these days though).