Pretty much the only classes I have found that don't actually work well together are Con+Con, and Arc+Con. Conjurer doesn't work well for tanking, and doesn't have a high damage output (if you are fighting Gnats, Con+Con is great however...). Arc's problem in this is that they can't distance off well, as they will be tanking until the Con has to cure them, at which point, the Con is getting spanked... Either way, you tend to be spending too much time in Damage control.
But, if you are looking for best performance in a duo, hands down Thm+Thm takes the cake. Alternating nuking with Fire can take out ~8 mobs (max number of mobs hit by a single AoE), and possible even walk away unscathed.
War+ Anyone else is also really good, in that war can take a hit, and dish it out fairly well. Add the two AoE abilities they have, and you can start getting decent chains with them. You can also Double Team War to be in this group as well
Gla+DD are also nice. Once Flash gets it's AoE (and Gla gets War drums), Gla can pull in a large group of mobs together, and hold them, while DD sits back and kills them. Gla does need healing abilities (prefered abilities would be Second Wind/Cure/Protect/Stoneskin, and never underestimate Raise).
Any two DD classes (Pug/Drg/Arc) can also work, but you will not be able to kill as fast as the other groups (Thm and Gla get higher mobs, War, mobs would be at the same level, but would not be killed as fast). On a side note, Arc is always better w/ another Arc in this case, as they can keep distancing off of each other.
Thm's, and Gla can take on the highest level mobs with the lowest chance of death. The War and DD you have to scale back the level of mobs by a few levels to reduce the chance of death.
Thms and Wars can kill groups of mobs faster than Gla+ or DD team-ups can. While Gla can pull a group, and the DD may have a few AoE attacks, they don't compare to the other two.
Thm's easilly have it hands down, While Burning MP can be painful, the refresh in the game gets you back on your feet quickly, as well, you are able to kill the highest level mobs in the fastest time. And Ideally, as a Thm group, you never fight a group of mobs that can't be killed by 3xFire+2xFira (both cast Fire, both chain to Fira, if necessary cast fire again). Fighting doesn't get easier than that (other than Cons and Gnats... Sorry to keep adding that, but the damage Stona does on gnats is just sick and wrong).
But, this has just been from my experience...
On a side note, I didn't add the Con+* but, yes, with a Con, you can use any class out of the box, no cross class skills, and it is the simplest, and an effective party (but keep in mind Con+Con REALLY is hard in comparison to almost any other pair-up).
Edited, Jun 29th 2012 1:11pm by rfolkker