The school that just accepted you off the waitlist should have communicated how long you have to accept the offer (usually this is in the offer letter or email). You have until that window closes to make a decision, but since it is after April 30th you can't accept the waitlist offer without withdrawing from the other school per AMCAS guidelines.
I was in a similar situation recently, but thankfully I'd already been notified of the financial aid offer from the first school and the school that accepted me off the waitlist was sure to send their offer within 2-3 days.
Hope this helps!