Committing to one school by May 1st, then breaking it later when you get off a waitlist is totally fine.
Depositing to two schools before May 1st to keep your options open is not fine. You may be rescinded for doing so if one or the other school finds out.
I agree that it is highly unusual for a college to give additional financial aid after May 1st. Are your referring to an application for an outside scholarship? If so, I would commit to the school you can afford without the scholarship, as outside scholarships tend to be highly competitive. The best outside scholarships will give you the information you need before May 1st as well, but some do not.