for just dualboxing you could go with any 2 characters, but you usually want them to compliment each other:

Warlock + Shadowpriest
Warrior + Healer (shaman for windfury, druid for instant hots while running)
Hunter + Disc Priest
Shaman + Shaman for more and different totems to use
Rogue + Feral Druid

Some are harder to /macro or match, but isnt that what all the fun is about ? research and trying.