How do you plan to play?
Before I got 5 accounts I had only 2 and my favourite duo combination was/is without any doubt hunter + priest. Using a tank pet (like a bear) and that trio can handle everything (at least outside dungeons).

You do mention dungeons though. How will you play those? Filling up rest of slots with pugs, or playing with a duo when you outgear the dungeons slightly? If the second alternative is your idea a hunter + healer combo will handle dungeons nicely as well.