Paladin tank for sure...

The key thing about paladins when dualboxing is they are easy to tank with compared to warriors(warriors are basically doing something every global cooldown.

Then for the healer I'd do - Shaman, or Priest, or Druid

BTW, Paladins can AE tank very well... Add in a healer for tons of fun

Edit:

I missed this part..

I'd personally love to do druid / druid because I've always wanted to level druids. However, with all the shape shifting, I'm not exactly sure how I'd pull it off. Or well, how easily it'd be to pull off. I could just be psyching myself out too. I'd also love the HoTs instead of relying on straight single heal hits.
Then do druid/druid

1) You like the combination
2) It is playable without to much effort (aka, not like 2 rogues)

Shifting isn't a big deal. Unlike warrior stances, you don't change stance much at all in PvE (a lot more for running in PVP)

For example, if your tanking your in Bear (not Bear then Cat then NE then bear again... It's just plain Bear) If your DPSing, your in Cat unless you get an add on you and need to tank or something... If your healing your in tree, unless HoTs are not enough for that fight, and in that case your a NE.

Some things to think about...
2 druid = both characters have stealth! (sneak to bosses and assassinate them)