I'll add one point based on brokebuthopeful's post solely for clarity. If you accept school B offer in 5 minutes (and not 48 hours), you must then withdraw from school A immediately. Granted, I would personally wait until the acceptance is noted in the CYMS tool and you can select PTE before withdrawing. School B will have to do the 'paperwork' before you can select them as PTE. I've had one school take a day and another take a week (though it will likely be quicker after 4/30 as schools want to figure out their class). School B would likely not withdraw your acceptance when they see both schools (well, maybe after a few days they would call or something) , but school A likely would.