I dual-boxed a Warrior + Holy Paladin combo to 70 and it was all kinds of awesomeness. Warrior was 100% Fury and did almost all the damage. The Paladin used the talented +armor aura (Devotion Aura?), which helped mitigate a lot of damage on the warrior (usually in zerker stance), and you don't really have to worry about getting healing aggro either, since the Paladin also wears plate. You also get the Might blessing for your Warrior, which really adds a lot of attack power.

It was incredibly easy churning through normal quests. Also, I think I duo'd all the 5-man instances up to Sunken Temple, at which point it started becomming too difficult. I also failed on getting to the hydromancer/gazrilla in that Tanaris instance (I wiped and said f-it), but I was really surprised at how far I got in the 5-mans with only 2 characters. Most of the outdoor 3-5 man elite quests in Outlands were also doable with this combo.

Super-easy combo and fast leveling via quests. One of the other nice things was that if you wanted to do something that's too hard to duo by yourself, it's very easy to fill out your group with other dps players, since you already have a tank + healer. You'll impress a lot of players too when they find out you're doing both tanking and healing.

P.S. I also tried a Priest+Rogue, Feral + Resto Druids and Enh+Resto Shams up to level 25-30 and they were nowhere near as easy and efficient as warrior + paladin. The two Shamans were fun, though.

Rogue/Priest was kind of irritating, because you really need the rogue as your main, but it's innefficient a lot of times. Stealthing and attacking first with Rogue is kind of a waste of time, so you'll feel like stealthing and pulling stuff to you with the Priest. But if you do that, then the Priest sometimes gets such a beatdown that you're too busy healing and you can't concentrate on controlling your rogue. So in the end, you just want to run up to the next mob on the Rogue and say screw those nice openers. The Rogue is just such a busy button masher class, that I don't like it at all for dual-boxing.