Mix 2 Takes Together- NOT Blend Transition

I have a pre-recorded performance from an actor where he recorded his voice and video of his face.  I'm using "ManyCam" to create a virtual camera that Character Animator can see and respond to as if it were a live webcam feed.  The problem is that his performance looks a bit odd as a stand-alone movement.  I'd like to keep what he did in a subtle way, but overlay my own performance on top of it.  Is there a way to mix these two together- not just fade (Blend) from one to the next?  It would be great if I could, say, use his performance at 30% and mine at 70% and just average them together.