My main 2-box is shaman + druid, which works pretty well. Actually, shaman + pretty much any class works pretty well, since shamans are very much a generalist class. Essentially, when I duo the shaman/druid, if the mob's slowed DPS exceeds the 300/tick regen from Quiescence, I use him as a supplemental healer and toss DoTs on the mob when possible. If the mob's slowed DPS is low enough for shaman self-heals to keep up, I use the druid as pure added DPS. The nice thing is that he's flexible enough to contibute nicely with excellent DoTs if my strategy is root/rot. Or he adds even more DPS using nukes & rains & dmg shields if I'm tanking the mob.
That said, I've been bringing along an SK on the second box for those cases where you really do want a plate tank on the mob rather than having the shaman tank it. Shamans have crappy natural mitigation and almost no defensive skills, so are really crappy tanks compared with plate tanks. SK bring some key things to the duo: snare, FD pulling, good plate tanking, pretty solid DPS, excellent agro.
My main duo partner is a ranger. This duo works well also, with the ranger bringing a bit more damage, slightly better agro, but tanking not quite as well.
I think if I were creating a duo from scratch, I'd strongly consider making a "2 pet class" duo, the best of which would probably be "magician + necro".