Even in heavy crew enviroment on B747 there is still only one overall commander, the other Capt is designated as P1 for the duration that the actual commander takes rest. The commander has signed the tech log for the a/c and is therefore responsible for it. Take this example and apply it to that of any a/c with 2 crew. Only one of them signs acceptance for the a/c and is therefore responsible. I would suggest that any change of command for our legal friends would have to result in another signature.