Dave is certainly correct, of course, that you can access your multiplayer opponent's side by running the save on two machines, or in two instances. You need their password though, no?
I think it really should be up to the player to decided whether to cheat or not. We're all (sadly) big grown ups now. I second Erik's request as ESPECIALLY WHEN TESTING BETA FEATURES, as many of us do and have done by putting together small scenarios, and as is of assistance to the product, it is extremely annoying that you never know the truth about the opponent's position - ie how many tanks etc did you knock out? Even the surrender picture is still subject to FOW. I also think that you should be able to switch sides and this would present very enjoyable and interestingly enhancing capabilities - let the AI pick up from where you left off etc. It would let you test various defensive options etc...
Either way, I think if the worry is that MP will be vulnerable to cheating then another solution could surely be found to that? And, as I said, don't you need your opponent's password to access the save as the opponent?