Is there a reason you want to complete your double major other than it being an accomplishment? If you're wanting to go to PT school, I don't see why you would decline an offer to go to PT school. Do you have awesome stats and know you will get in somewhere, possibly not the same school, next year?
PT school isn't easy to get into, so I personally would never decline an offer just to finish a specific major, but I don't know your situation. I can't really speak to the applying-to-the-same-school part of it, but my gut says that doesn't seem like they would consider it a great move.