If you purchase through Disney, the two contracts will have the same use year and membership number. At 7 months, they will work together as a single contract for reservations. At 11 months, only the points from each respective resort will have home resort priority.
If you are adding on through resale, be sure the contracts are titles exactly the same way, and have the same use year. Have your resale broker/closing agent let Disney know that you want the resale added to you existing membership.
If add-on have a different use year, they will be a totally separate membership. You can not combine points for any single night of a reservation, but you could reserve some nights from one contract, and other nights with the other membership. Hey would have two different banking windows, etc.
Unless you specifically have a reason for wanting different use years, it would be simpler to stay with a single use year.
But, other than the 11/7 priority window, having multiple resorts isn't a problem, in the same membership and use year.