Lichloathe actually touched on this topic fairly well on the DK Forums on his stickie... Though I can't quote it word for word, I can say that as far as it's performance goes...
A single wielding 2 hander is generally going to outclass a duel wield tank. There's a couple reasons behind this. Coming from a threat tank, my biggest pet peave with duel wielding is weapon damage contributes to a very very large portion of our threat. That being said a 2 hander is going to hit harder than a 1 hander. Thereby producing a higher overall top end TPS (from weapon damage)
As far as frost goes, I know there is quite a bit of magic damage thrown into the mix from Howling Blast (and perhaps frost strike? with a small amount of extra frost damage) However even the FS runic power dump is based on 60% weapon damage +Frost Damage.
Another good reason that a 2 hander is generally going to outperfrom the duel wield is the Parry Haste Factor. Wherein when a mob parries your attack they gain a slight haste to their swing timer. I believe it is 40% if I'm not mistaken. Unless the parry haste would reduce the mob's current swing timer to less than 20% of full duration.
So basically when you duel wield you are doubling your chances to be parried, thereby doubling your chances to recieve a parry haste attack... taking more incoming damage, and placing more stress onto your healers.
Now I'm not saying that it CANT be done. A lot of people have proven otherwise, I'm sure it can be done, but it would be very dependant on gearing. Wherein the hit cap for a single wield melee is 8%, a duel wield is 25%.
264 Hit Rating vs. ~891 Hit Rating.
And to truly avoid recieving a lot more incoming damage you'd really have to focus on hitting the expertise hard cap of 57. Or at least something higher than the standard 26 for it to be truly more effective.
I imagine that with the correct theorycrafting and numbercrunching it could perhaps actually do better than a 2 hand DK tank, but that number crunching is well beyond my ability. And for someone starting out into DK Tanking, it would be well beyond their own ability as well to make it truly efficient.