Job Dilemma

[twinnurse002]

So this week I received two phone calls on the same day from different hospitals for interview. Hospital A interview as on Wednesday and everything went well and was offered a position by HR by phone the next day. They gave me 24 hours to make a decision. On friday morning I had an interview with hospital B (the big hospital that I really want to work at)and i believe everything went well and she said she has a couple more interviews to do and i wouldn't know about anything for a couple of weeks. So Friday afternoon I called hospital A HR and accepted the position because who knows how hospital B would work out and I need a job. Hospital A orientation starts end of July and hospital B starts in september. My question is what if hospital B calls like couple days before I start orientation for hospital A. Would be be ok to call hospital A in such a short notice that I have to decline their offer and if i do who do i call and what do i say? Is there a way to ask a hospital to do paperwork to secure a spot?