You can solo ANY char, so you can duo any 2 chars. The biggest part is teamwork. You probably want a healer, just makes things so much easier. You probably want to match timings with attack between the two. Imagine a mage that needs to cast a 3 sec spell and a warrior that uses an attack every 1 secs. A lot of the time, the mage will be spammed with keys. Now imagine the reverse, a druid that casts a spell every 2 secs and a DK that casts every 10 secs. Depending on who is the main, if the DK is the main, then the druid is starved for keys since the DK doesn't need to use them. Another part is movement. Try to pick a duo that works with the same movement. A melee moves a lot and would break the casting of a caster. A hunter and lock can move while fighting, but maybe (just guessing) a shammy/mage would want to stand still. Just try some out and see how it works, you will probably notice right away what doesn't work.

Also, DON"T just listen to people about what works. You will have to live with these chars for a long time. Pick ones you like, not what other people tell you.

Good luck.