I've split up instancing with a friend numerous times; he 2-boxes a feral (tank) druid and hunter, while I 3-box shammies or run 1 mage/2 shammies. We're both fulfilling two roles; he tanks and mashes a dps macro, I dps and heal as needed. It seems to work out pretty well, particularly if I drive from the healer (and my friend drives from his tank). DPS is the easiest thing to relegate to a mashed macro on a follower; this won't break any dps records but for 5-man content it'll sure get the job done.

Note that shammies seem to be the only class that can spec for ranged DPS and drop a heal from time to time with no penalty. SPriests and boomkin have to drop out of form to do so, causing the use of a GCD and mana to switch back, or resulting in a sizeable dps drop if staying out of form. Ele shammy healing is still pretty potent, just not very mana efficient.