I second the boomkin + elemental shammy vote.

I'm currently dual-boxing (non-RAF /cry) a prot pally/holy priest duo up to 80, once the pally is 80 the power of my existing 80's bankroll will get her def-capped right away. Then I'm going to RAF a 3rd account from my 2nd account (pally is on the main (1st) account) and RAF-instance boost all the remaining classes to 60.

Once that's done I've already planned out my dual-box teams to dual-box from 60-80 and elem shammy + boomkin is one of them:
1. rogue + warlock
2. boomkin + elem shammy
3. warrior + mage