I didnt 2 box this but I did level like this with 2 people playing the characters.

Feral Druid + Disc Priest.

You have awesome damage in your cat form and the Disc priest can put out the DPS as well as pull off great heals when needed. Most fights just dump a shield on the druid before you pull and it wont even take any damage.
Druid can easily switch to bear form for world bosses/group quests and you can 2 man them with the priest doing some damage + Healing.

The feral druid is good as there is no mana to worry about (For the most part) essentially gets 2 specs(Tank + DPS) for the price of one, and the priest can just trail along behind to recover mana if they run out and none of their mana recovery's are available.

Other combos(In my personal order of preference) based on this are to replace the Druid with
Pally (Although mana based has great mana recovery)
Warrior


Replace Priest with
Druid (Boomkin, but could still do some decent heals when needed)
Pally (Would have to be a dedicated healer or you need to do 2x Melee chars, which is more then possible just a bit more involved)


Let us know what you decide on and how it goes.