My first dual-boxing experience was when BC first hit. I had a Holy/Smite priest and a Prot pally. I actually just used 2 keyboards and 2 mice and had no problems. Once you get Holy Nova on the priest, and Consecrate on the pally it becomes almost trivial to level. Run in with the pally, consecrate, and then spam Holy Nova. Drink, repeat. As the pally and priest level it becomes easier.

Jusy my .02