If you purchase an add-on at the same resort you already own, then those points are combined at 11-months.
Example: You own 160 AKV points and later do a 50 point AKV add-on. This is treated as if it were a single 210 AKV contract. While
DVC will use the points from the two contracts separately when making reservations, this is transparent to you.
Suppose you make a 180 point reservation. DVC would take the 160 points from your first AKV contract, and 20 points from your 50 point AKV contract. But that's internally within their computers. As far as you're concerned it was a 180 point reservation made using your 210 AKV points.
As others have said, if they are different resorts, then points cannot be combined at 11-months.
At 7-months points can be combined, and you will need to tell MS which points you wish to use. Example instead of the above, suppose you have 160 AKV points and 50 SSR points and want to make a 180 point reservation at BWV. You could use 160 AKV points and 20 SSR points, or maybe all 50 SSR points and 130 AKV points, or maybe 40 SSR points and 140 AKV points, and so on. Any combination you want. MS will not automatically select points from one contract or the other. You must tell them what points you wish to use.
You should plan ahead when making these decisions. In the above example if you know the following year you want to go to AKV during a popular time, you should use the 50 SSR points and 130 AKV points for your BWV reservation so that you can bank the remaining 30 AKV points making them available for an 11-month reservation next year. I.E, combine them with next year's 160 points to have 190 total points you can use at 11-months.