You know since most of the Ret Paladin's (if not all) arsenal of abilites are considered melee damage, even the "holy" damage dealing abilities, I'm not sure Unholy DK will actually boost their damage output. I could be wrong on this though.

However that being said I would still have the DK tank for both the reason that the extra Ret Paladin offers an additional Divine Storm for passive group healing, instant FoL's with Art of War, and Lay on Hands for a quick emergency heal, as well as the fact that runic power abilites aren't exactly the easiest to macro into castsequences the same as a warrior's rage which means you won't really be using a DK follower to their absolute max potential. It'll be easier to get the most out of your DK by having him tank.