Actually, I think they sort of do, but not. See, the 10% proc chance in the first place only accounts to a bit less than half of your basic attacks. Having two such daggers technically does double the chances for basic attacks.
Abilities, however, I think are based on one dagger or the other. Flank Attack, for example, is based from the off-hand dagger, taking damage and procs from the dagger equipped in the off-hand. Twin Fangs, conversely, may count each dagger as a separate point of damage, and may then proc each strike separately. That one seems a bit unlikely though. Have not tested Twin Fangs enough to know. I can say that having Gift of the Talons in the main-hand never proc'd Hidden Blades with Flank Attack on the Assassin, versus off-hand that did.
My point is that, while they don't stack, I am pretty sure they are counted independently.