If you purchase directly thru
DVC, you will have the same Use Year and the 2nd contract will be added to your current membership.
If you purchase a resale with a different Use Year you will have a 2nd membership number. There can be an advantage to a different Use Year if it allows you more flexibility regarding when you will travel. There is minimal (if any) additional effort to manage multiple Use Years. We have 3 currently and have had as many as four and have never lost single point in 19 years. There are some good (free) programs to help track your points as well as the DVC member site. Even a simple checkbook program, like quicken, can easily track DVC points for multiple resorts and Use Years.
If you find a contract that works for you with a different Use Year don't let it slip by for that reason.
Enjoy!