Although I run as a blood MT I will share my "understanding" of DW Frost Tanking.
Runes - So becoming a DW tank means you loose out on SSG (25 Defense and 2% stam) but gain 4% parry that does not succumb to DR. So initally there is going to need some gear shifting to make it back to the crit cap of 340 unless you have 2 1H weapons that are heavy in defense or are just comfortably over def cap.
Threat - I am still yet to see any resounding numbers suggesting either is better. I'm pretty confident though in saying that 2H will come out on top simply because our main threat ability will hit harder with a 2H then our 1H. But there is the arguement that the added avoidance from DW will create more RS which brings the gap a little closer. Would be good to see some DW vs 2H numbers on Oblit/FS. Also remember our 2H will no doubt be tuned for DPS (slow big hit) where our 1H are tuned for tanking (fast small hits).
Avoidance - Obviously the main reason to go DW, by picking up 4% from runes alone the tanks stats on these weapons will help put more and more avoidance on the table.
I am considering running frost 2H for a bit and then possibly trying the DW option just so I can know personally how it weighs up, when I do ill put my findings here.