Dual boxing with Paladin, What other class should I choose?

Hey all, I'm going to reroll and start dual boxing and I'm really interested in my main being a prot pally, Im wondering what to choose for a second class. I was thinking a priest because I've always wanted a preist, and then when I would like to instance I would only need to find 3 dps since I already have the tank and healer roles filled :). I'm just nervous because I'm not sure a priest and paladin would put out enough damage to be a viable group for questing and such. Please post your thoughts.

For out-of-instance grinding at lower levels you could roll shadow/ret and just dps for the most part - once you get the talents/levels and want to hit the instance scene its easy enough to turn that into a holy/disc + prot combo which is nice and sturdy.