As a tank, you don't want the big nasty mob to Riposte your attack.
Dual-wielding is more attacks, so more chances for it to Riposte.

If you're doing heroic level content, and not raids, this probably isn't a huge issue.
If you are boxing 4 DK's, each attack has a chance to generate a Blood Worm.
So dual-wielding is more Blood Worms out and about.

There was a video a while back.
Four Blood spec DK's, in Blood Presence.
With a Priest.
All of them were PvP geared entirely.
And they did a heroic successfully.

If you want to run four tanks, you won't beat any enrage timers.
And trash will take a long time to kill, etc.
But you won't care who has threat, and it will never be your healer.
The team will have massive survival.

If you want to run one tank, you'll probably be better off with an "optimal" spec, gearing, gems, enchants, etc.
And have a healer (pve) spec for your healer, if that differs from your PvP spec.
And then take your other DK's and run them as Frost.
You'll kill a lot faster, maybe twice the DPS or more.
But all of your non-healer toons are in melee, so they're subject to PBAoE or Cleave effects, ie all the melee crap to deal with still.

Either approach will likely work fairly well.