I know if you transfer a toon (a DK) to an account with a DK, you can have two DK's on the same faction/account.
My roommate did this.

I'd likely go with 4x DK's and 1x Heals.
Paladin/Priest are most typical, if you're running arena.
But every healer is a good option for battlegrounds.

5x DK's will have some healing in MoP.
Conversion will be steady healing over time (perma HOT).
Or you could get a 50% heal, on a cooldown (same talent tier).
You can also Lichborne every 2 minutes (if you take that talent) and DeathCoil yourself while it is active.

Any healer toon will be far stronger healing against focus fire on one of your toons.
With a healer on the team, Purgatory instead of Lichborne will be an option too.