I put my priest talents in the Holy tree initially because I felt it would be too difficult to control the shadow abilities (Mind Flay, mostly) while spamming with my Warrior. After some experience, I found that the Holy tree did much better DPS than I first thought it could.

My primary attack button (#2) would cast Smite with the Priest, and Sunder Armor and the Warrior. My secondary attack (#3) would cycle between SW:P and Mind Blast on the Priest and Revenge on the Warrior. It would be a lot different on a non Prot-specced Warrior, and I don't know how much DPS you could do with this dual-box combo. My F1/F2 keys would be used to cast heal on the Warrior/Priest respectively

Another thing--at such an early level, it's really hard to tank multiple mobs since your gear is likely going to be poor, and the heals/mana pool won't be able to keep up. Once you hit around 20 or so, and get some decent Deadmines/WC gear, then things really start to look up.