I have a pair of level 74.5 shammies that I enjoy pretty much. The one in Front bears the brunt of the attack, and is spec'd Enhanced, with mix of caster mail and melee mail. I try to pick as much stamina as I can get. The one in back is Elemental, with caster gear and emphasis on mana pool/mp5. It has worked out pretty well with the Front guy being a herber, as that extra heal comes in quite handy.

Here was my layout/macros at 60:

http://www.dual-boxing.com/forums/in...&postID=142726

ELE Shammy:
http://www.dual-boxing.com/forums/in...&postID=142726

Enhanced Shammy:
http://www.wowarmory.com/character-s...&n=Sham%C3%A6n

I have a rogue/druid pair that I play. Seems to work pretty well. I also have a pair of death knights and pair of hunters. I have even tried shammy+hunter a bit, which was hard to setup what did what btw. Of course, that was back when I was just beginning, perhaps I should revisit it now that I have been doing it a couple months.

Shammies are the favorite for me. Some other combo might be better or faster, but the shammies have been easy to level up, and fairly intriguing to play and learn dual boxing with.