You are allowed to have up to two active waitlists per membership. Each booking category requires a separate waitlist so for example, if you waitlist for a Lake View studio and a Standard View studio at BLT, that is your two. If you just waitlist for one category of studio at BLT, then you still have one more to use at PVB (assuming you aren't waitlisting for another trip at the same time).
You can't prioritize the two waitlists (if they are for the same dates). If one waitlist works, then the other is cancelled. You can re-enter the cancelled one if that's what you really preferred, but you would go to the bottom of the list for that request.