Back before I found the joys of shared keypresses I used to two-box (manually - 1 keyboard/mouse for each client) a mage and priest in my raiding guild. I found that the priest which was my main at the time, was very very easy to heal with to the point that I rolled the mage to have something else to do.

Since then I've found that for single target healing a holy specced paladin is surprisingly easy and very resilliant, for 1-key group heals the priest is great but squishy if you over-aggro.