I have a couple 2 man teams... I used to do 5, but got expensive...

I have a bear / priest group - my druid is my farmer (so I will level her first in MoP) and my priest is atonement. Its really easy to box since the priest is smite healing. I drive with the bear...

I have two pally's and two mages. I have been doing daily's with pally tank / arcane mage... its really easy. I just recently have leveled the second group.

I'm thinking about leveling MoP pally / pally and mage / mage but haven't decided yet. I might keep them pally / mage depending on how scenarios work out in MoP. I've done the LFR in Cata with my priest smite healing and my pally as dps... Not too bad although the RNG of the Ret Pally spec right now doesn't put my dps anywhere near the top. I haven't dualed my mages yet as they are not geared similarly - one mage is in 378's while the other is sitting in some greens.

I've been messing around with them as we get ready for MoP - I really can't wait to get on beta to see what will be best...

I read all the time that there is synergy using the same class and dual boxing heroic 5 mans is probably easier... But pally tank / arcane mage is quite a bit of fun... You just gather everything up and let the mage burn them down... its really fast. The tank can take the damage and cast a word of glory if needed and the mage never get's touched..I don't know how much fun leveling the same group up twice will be in the expansion tho...