I believe that a condition of acceptance is that you do not hold a place at another institution. So, accepting both places will, in all likeihood, make both offers null and void. Remember, what you are accepting is a contract, like a job. Of course, if things come up, you can change your mind about a place, and people will probably understand. But nobody likes a time waster (although I know you're not doing it on purpose). The place that you end up rejecting will be annoyed, and its not great to upset people who might be useful to you in your future career! Bottom line anyway, you're not allowed.
B.x