I've duoed most of my way through the game, and one of the combos was mage/shammy. It worked pretty well I think. My pet was the tank, and got buffed up quite nicely by my friend. Only trouble we ever ran into was with adds - no CC, except for off-rooting. I remember it being fun. Not sure of the loot getting part, though. But it was a lot of fun.
Basically, as someone stated somewhere on these boards, any combo works - you just have to come up with the right strategy - but some combos are better than others. If you both like those characters, you'll enjoy it.
Good luck with it, and please let us know your experiences! :)
Edited, Feb 19th 2008 4:44pm by Shiqra