DK and Warlock

My fun is to discover and level - slowly, probably forever - all classes thru duo or trio (sometimes 5 for instances). I like the synergies of my duo (warrior, priest), (hunter, shaman), (druid, rogue), (pala, 2 mages).
Unfortunately, stuck in the HP inn, are my DK and affli warlock. Not sure there is great synergy between them. During fights, the gnome will stay in the backpack of his human DK and throw a few dots.

More for fun than for synergy but would you advise a specific tree for the DK ? frost, blood or unholy ? or it doesn't matter ?

Thanks for your help !

i'd gather up a bunch, spread diseases, Howl of Terror.
bandage up, wait for the feared mobs to attract some adds.
continue the hack n slash.

yea the warlock can easily add a curse and a corruption. no need for anything else really.

Well I run with a DK and 3 warlocks plus priest healer on my team and one of the best synergies you can have is ebon plaguebringer deep in the unholy tree. It will give you and your warlock 13% more damage for all your magic attacks and free's up Curse of Agony to be cast on your warlock instead of a CoE. If you are really commited to say the frost tree then spec your warlock for felguard so your pet will get the nice melee attack buff from Icy Talons. For just duo'ing running around (non-instance) I would imagine you would be hard pressed to beat a Frost DK and a Felguard Warlock for trash DPS. For instances you would be hard pressed to beat a Unholy DK and a FG/emberstorm combo.

Allesarelle