If you have two contracts with the same UY, even at different resorts, you'll have only one membership. This means that you'll have only one account and when you log in you'll find all your points.
This way it will be easy to combine points to make reservations.
If you have two different UY, you'll have two different membership, meaning that you'll have to different accounts to login into members website and make reservations, making more difficult to combine points for one reservation.
To combine points you have first to transfer them and the system have many bugs with transfered points. You'll probably not be able to see the transfered points and you'll have to call MS to make the reservation. Or you might also end with lost points and have to deal with MS for days to recover them.
Example:
You have 100 point for AKV with a December UY and 100 points for OKW with a December UY. You have only one membership.
Now you want to book one stay at BLT that will require 150 points. You can login at the 7 months window and make a reservation online.You will be able to use the 150 points and the system will ask you which points you want to use first: do you want to use 100 points from the AKV contract and 50 from OWK, or 100 from OKW and 50 from AKV? Or maybe you want to borrow points from your next UY from the OKW contract and keep all your AKV points to make a different reservation in the future?
All done online and very easy.
Instead, if you have 100 points for AKV with a December UY and 100 points for OKW with a September UY you will have two membership.
To make the same reservation you can login into with one of the membership and book all your stay from one contract borrowing points.
If you don't want to borrow, you can book the first days with one contract, then wait a few days until the 7 months window will be open for the remaining days and log in into the second account to book the remaining days. If the days are no more available (can happen) you'll be out of luck, you'll be able to create a WL.
Or some days before the booking, you'll call MS to transfer 50 points from the AKV contract to OKW one. Since the UY of the points are different, you'll log in into the OKW account and you'll see only 100 points. The 50 points are there (probably) but you'll have to call MS to make a reservation. And after calling, the CM may not find your points anyway, so he will have to call a supervisor to sort it out. It may require some time and in the meantime the rooms that were available may be booked.
As you see, having the same UY makes the whole experience much easier.
I think that if you plan to buy two big different contracts that you'll use each on their own for different bookings at the 11 months windows, it does not really matter.
But if you add on a small contract, the same UY will save you a lot of headaches.