I have that situation. My BLT and SSR are June UY, same membership. My BRV is Sep UY, different membership. As JETSDAD said, you can't combine the points in two different memberships unless you do a transfer. A transfer is not always a great thing either, because you can't transfer banked or borrowed points. It's best if you use them independently. This is new to me because BRV is a new contract as of last year. I used both memberships last February for a split stay between HHI and SSR. Right now I have April booked Split stay BRV then BLT. At the 7 month window, I plan to combine my BLT/SSR points into one reservation. But the BRV reservation will remain separate. However, if I decide to lengthen my BLT stay instead, I could book the same class room at BLT using the BRV points. Then I would have to ask MS to flag it as a continuous stay, and ask the front desk not to move us if possible. Chances are I would not have to move, but it is still will remain a separate reservation, even if I don't move, they can't combine two different membership reservations into one.
If I could have the same UY I would, it's better. But when you're trying to get a great deal on resale, it broadens the opportunity to find a great contract, if you don't mind the drawbacks.