I have put this idea on the backburner and spec'd one druid a tank and one a healer. I may try this down the line, but that will likely only be if I'm bored and seriously outgear the instance.

I'm finding out that a resto druid kicks ass. Wild growth is much better than chain heal, being able to heal while running is nice, though missing fear protection is a huge disadvantage.

I'm finding out that feral druid tanking is decent, but nothing compared to the easy mode of a pally tank. The nice part is that everyone has fairly high armor (for non heroics) so if a mob goes after a moonkin or tree then they can take a couple hits and wild growth helps. Also, 3x typhoons gets them off the group and back to near the tank pretty nicely.