To the OP:

Warlock + S.Priest work well together - a lot of spells with the same cast time, and both deal shadow damage primarily.
Dest. Warlock + Fire Mage has strong synergy.
Demo Warlock + BM Hunter could work well; hunter pet has taunt on (its the much better tank).
Tank + Healer is an option - DK has highest dps of the tanking form tanks.. add in say Resto Shaman - Windfury Totem, Strength of Earth, Healing Stream Totem, Flametongue Totem... E.Shield the DK and W.Shield the Shm.

Almost anything will work, with two toons. I like the idea of different toons if you're doing just 2.


Addendum:
Well shaman's have fast cast spells, which means you get a smaller portion of your spell power applied to each nuke.
Per 3.5 seconds of untalented cast time, you get the same as anyone else.
Naturally, with less bonus spell power per nuke, you'll need to nuke more often to deal your damage.

Boomkin scale better then the Elemental Shaman, but the principle is the same with Wrath.

Slower cast spells such as Fireball or Shadow Bolt get a larger coefficient per cast, but still have the same amount per 3.5 seconds of cast time.
If you want to drop things in one cast/volley, you need either slower casting spells or more casters to cast simultaneously.