I usually monitor closely in these situations. If one clears you can then decide if that is your first choice (in which case, call to cancel the other wait list). If it's your second choice, you can just leave the other wait list in place.

If they clear within a short time of each other I'm not sure if the system will keep both confirmations temporarily or the second clearance will override the first.

AFAIK, an agent needs to accept the wait list clearance on your behalf (on the phone or at the airport) in order to be confirmed on the new flight. It does not automatically replace the previously confirmed flight.

Charles