I think this is both a good and horrible idea.
On a strict itemization standpoint, DPS 1hander are currently our most sharded epics since most melee DPS class are using 2handers right now. It would be nice to get more class to use those.
The problem is that for a DPS class there will always be one optimum spec that slightly outperforms all the others on either DPS or utility(or both when you are a survival hunter). So adding a duel wield option will either make all DKs dual wield or none of them since the spec will either be the optimal one or 2handers will still be superior.