It simply doesn't fit the theme of the class, and the fact that Frostmourne is being split into two separate weapons pisses me off.
Frostmourne was shattered, sure, but it was shattered into a whole helluva lore more than two pieces, let alone two pieces that you could slap hilts on and start swinging. Clearly, some amount of reforging is going on, so why couldn't we reforge them into a single weapon, just like the Frostmourne we've all been eager to wield? Is it not the PERFECT embodiment of the Class? If Paladins are wielding the Ashbringer, and Shamans the Doomhammer, then why are Death Knights given the shaft? Since all of Frostmourne's souls were expelled, would it not serve as a perfect runeblade for our goals?
Look, I'm fine with having the OPTION to dual-wield -- but not at the EXPENSE of two-handed Frost, let alone at the expense of Frostmourne itself. Instead, we get to one-handed mockeries of that weapon, which really does feel like a slap in the face.
Blizzard, if you read these boards, I urge you; please reconsider. Maintain the ability to choose between one-hand or two-hand Frost, or simply remove dual-wielding and be done with it; there are enough dual-wielding classes anyways, and nothing about dual-wielding ties even remotely into the "class fantasy" of playing a Death Knight.
And if there's any kind of concern about "too many Artifact Swords", then just replace the Unholy Artifact with something else, such as a large, powerful-looking scythe (think "The Harvester" from Darksiders).