If you add fun factor.

Boomkin duo seems fun. There's a video thread with a dual boxing boomkin in the video section.

I don't think 2x elemental shaman works as effectively as 4/5 shamans, since you don't have cc nor much anti-cc, you're better off going with a class that has some form of cc. So, dual mages should work well. If you're serious about PvP, undead is pretty much a must. Or maybe a shadowpriest/lock combo, also quite interesting.