Hello
I duo a lot with my GF. One of our duo is me a level 60 templar and her a level 60 wizard. We can duo pretty well, since templars can equip heavy armor i can sustain a bit of damage, at least long enough till she burns the mobs to the ground ! By duoing well i mean taking heroic mobs a few levels under us. There is no way i could 'tank' lets say, level 65 heroics mobs in a duo like that. I usually can 'tank' (i have no taunts lol) up to level 54 heroics 3 arrows up easy, higher than that needs to have only 2 or 1 arrow up or i die too quickly.
Our other duo is lower level, but even more efficient. I have a troubador and my GF has a fury (respectively 31 and 36) and its a really good combo because of bard's buffs/dps/avoidance combined with the fury's healing and dps (furies got nice dps for healer class).
I sincerly think that any 2 classes can duo really well, thing is, some will better than others, and even then, depending on situation. While a duo of Paladin and Templar would do good against undeads, it would be slower against other mob type. A duo of Guardian and Templar would have very high surviving factor but would take longer to get going due to low dps. I say, just pick the classes you find the more appealing :)