Points carry the home resort advantage so if you have a small contract at one resort, you will only be able to use those points at the 11 month window.
As mentioned, some using banking and borrowing to get more nights and then alternate their trips between the home resorts they own.
Contracts that have the same owner and same UY are part of one membership. That means, you can use the points together to make a single reservation. So, when you buy a contract, it is considered an add on because it goes into the same membership. When buying direct with DVD, add ons can be bought with as little as 25 or 50 points, depending on the resort.
If you wanted a different UY, and are buying direct, then it is no longer an add on, but a new membership, which means the smallest contract you can get is 100 points.
If you have contracts in different UYs, you have different memberships. It is like being two different owners. Points can not be combined into the same reservation without a transfer. And, you are only allowed one transfer in or out of your membership.....for the past 4 to 5 years, they had been allwoing owners with multiple memberships to do more than one, but that is no longer allowed.
If you buy resale, and get a different UY, it becomes a new membership. Many of us do have multiple UYS, but we also have enough points in each so they can function on their own....use for different trips, do split stays, or just book some nights on one, some on the other, and ask MS to link so we don't have to change rooms (which they don't guarantee, but they have never made us move).