Oh, so you are going to give me two hands to defend with? And that will ensure your success? Be careful with that. How will you guard against my legs?
Now, I am not the smartest person in the world, but not the dumbest either. A lot of any defense has a lot of variables. You may be much faster than I am. You may be stronger than I am. You may have any number of other advantages over me.
But what would prevent me from deflecting your punch just enough to make it miss, and striking you in some way? How about if as I deflect it while stepping in, I grab that arm with both my hands, one in the forearm, the other on the upper arm, gripping painfully hard, then as I continue to move in, all the while grabbing your skin and muscles, I turn and slip under your hip and throw you? That is a viable defense. But of course, there are many variables that will prevent you from countering it, or me pulling it off. But if I get it right, you are in trouble.