Platonist: I do know of someone in that situation in previous years. Practically speaking, most schools are quite understanding. In this case, the person called school B and asked them to grant him until the end of the day to hear back from School A. He then called school A,, and informed them of how he had an offer from school B and needed a decision on the waitlist as soon as possible. All in all, just needs some coordination.
The story ends, rather happily, with said person being accepted by school A. I imagine schools inform students that there are these things going on, so they have to wait. But a non-uniform deadline would be worse than a uniform one.
