If you add-on direct from Disney, they will only sell you points with the same UY month. If you want a different UY Month you would have to buy a new master contract or purchase a resale contract. By having all points under one UY Month and membership number, you can combine points to make reservations at the 7 month mark. If you have a different UY month, you are going to have an additional membership number. To combine your points, you would have to use your once a UY transfer of points.
For what it's worth, I have two contracts with different UY's (Feb & Oct) and it's not a big deal at all. I use Caskbill's point tracker and we tend to bank one contract and borrow on the other so we can make entire reservations at 11 month mark. Two use years has not been a problem but if all else is equal, it probably makes things a little easier.