I dual-boxed a mage and priest once. Always had the mage as the primary character with the priest as a healbot.

Dual-boxing can make you sloppy sometimes, at least with PVE. With a priest as a healbot I didn't kite mobs very much compared to when I played a mage as a single.

I always had the priest as Holy but probably should have gone Shadow and used VE more and could have been more aggressive with the priest rather than just healing all the time. They both made it to 70 before I started even looking at Shadow.