Even if you do draw one-handed, it's not a good idea to try to 'fend off' the attack with your off hand. In the classes I've taken, we always practiced one handed drawing, but you ALWAYS brought your off-hand back to your chest when you drew. Regardless of where your hands start, once your dominant hand starts towards your gun, your off-hand goes flat on your chest. That way you don't end up inadvertently shooting yourself in the hand. This is especially true when you are dealing with someone very close to you, because you'll be firing with the gun still tucked in close to your chest.
Just out of curiosity, have you ever tried doing this with a training partner and a blue or airsoft gun? If you have, and it works for you, then good! However, if you haven't, I think you might find that if you're close enough to the attacker to be able to fend off him off with your off-hand, and drawing a gun is justified, immediately ceasing an active counterattack while you get your gun out may very well be more dangerous than having your hand in front of the muzzle when you fire.
I've gone through a class where they had us put our offhand on our chest while drawing, too, but it was simulating our off-hand being injured or carrying something, and we put it on our chest just for range safety, not necessarily because it's a good tactic while actually in a gunfight. Just a thought!