Any tanking capable class + Any healer capable class I think will always go the quickest. But level them both as DPS spec and if you have the cash Duel spec at 40.
So
Spot 1
Arms Warr, Feral Druid, Ret Pally (I wouldnt really bother with DK boosting but you may want to switch once you hit 58)
Spot 2
Ele shaman, Boomkin, Shadow Priest, Ret Pally

You wont usually need the heals in battle but it will reduce your downtime. Pallys work well as you can just judge & Seal wisdom to keep your mana up so your downtime is almost 0.


you wont get many bites on your DK+heals pre lvl 60 as most people I imagine will just use the new dungeon finder tool which your DK wont be able to.

The other toon I would maybe suggest is a mage instead of a tank. You gain portals, food & drink. Which makes everything that little bit faster.