Druid + Warlock will definitely work well together.

For leveling, I'd consider speccing the druid Balance.
Even if you don't bother to manage Eclipse procs, it will be faster leveling then Resto.

Down the road, Resto + Warlock, will get you a pretty decent team for 2's arena if you want that aspect of play.
Or run them in battlegrounds together.

Pretty much almost any combination of two toons will work.
Quite a few would go with the Healer + Tank option.
A Felguard can easily tank anything pre-heroic, during leveling up.
Not positive on normal instances... it will likely survive, but not sure that it would hold threat.