Well don't wait for it to accept the other offer.
You can always back out of the other offer, but not if you don't take it first.
We've had people back out of offers they accepted the day before they were due to start when a better offer came through for them. No harm, no foul. It happens, and we wouldn't want someone on board stupid enough to reject something better out of some sort of weird sense of obligation anyway.
Now if they try to use that as a bargaining tool to get us to match it...blacklisted...permanently.