Dual-boxing question

[nodoze]

Single or duo cats should be very fast as good movement speed, good damage, & healing resource is separate from your damage resource...

I don't think Duo druids would be significantly slower than single druid as things will die faster with close to double damage plus you get some bonus XP which should offset at least some duplicity overhead.

If you are on a contested PVP server (or worse dominated by the other faction) Druids can stealth and level OK outside of instances...

If that isn't a big consideration then Mage+Priest can be pretty fast as well as you get free water/food along the way and even at cap your Mage can provide some usefulness via at least water+food for your Warlock+Priest at cap (and a buff on the Priest before switching the Mage to the Warlock).

Hunter+Druid should also be a decently fast team. Solo Hunters are very fast to begin with and have a built in limited Trinity and Druid only makes it stronger (MotW, Thorns, Hots, direct-Heals, extra DPS, etc).