I 2-boxed my mage and feral druid all the time in instances. I MT with the druid and CC/dps with the mage. Focus macros make the CC part easy, and spamming frostbolt doesn't take much attention so I can focus on tanking.

I've successfully done nearly every non-heroic 5-man and I've successfully tried heroic Mana Tombs, Sethekk Halls, and Mechanar. I've also done full Karazhan clears 2-boxing (tanking both red and blue beams on Netherspite was a challenge, especially with the void zones.) We had a warrior MT Prince so I could go kitty, and I ended up #2 and #3 dps on him .