So what’s the point of the PTE if you can have deposits paid for two schools? Especially if School A can’t even see that I’m paying a deposit for another school? Would AMCAS not see this?
The point of PTE, especially before 5/1, is that it forces people to give up multiple As, which is what creates all the WL movement around that deadline. After 5/1, it still forces people to give up multiple As, but only to the extent schools require them to make the election, as your new school surely will once it gives you the financial aid information you need to make an informed decision.
The only reason School A cannot see the new acceptance is because School B is not requiring an immediate PTE election. That will change as soon as they push out your financial aid information. At that point, you'll only have a few days to make a decision, at which point you'll be back to only having a single acceptance.
You are only in this position because you recently received an offer off a WL. The alternative would be to force you to drop your PTE school before knowing what the new school will cost, or not allowing you to accept the WL offer at all. Neither would be a fair result, so this is what we have.
With no PTE requirement or deadline, a ton of indecisive people would screw around until the very end, which would result in chaos as the bulk of WL movement would not happen until just before orientation. You are an exception because you received a late-ish WL A. You won't be holding two As for more than a few weeks.