Buy in a different use year and you will have two separate member numbers. You will be able to find both together in one online account on the member site but each is treated as a separate membership.
You will not be able to combine the points from both use years to reserve any single night absent a transfer. For a single trip, your options will be to either transfer points from one use year to the other before reserving, allowing you to use the points together, or, alternatively, reserve some nights with one use year and the remaining nights with the other and then contact MS and have it link the reservations. Note, that if you go the transfer route, it is easy to do via contacting MS but you lose the ability to make the reservation on line at 8 a.m. eastern because the transferred points will not be shown in the use year account into which they were transferred (that is simply a Disney computer inadequacy not a rule), and thus to make a reservation using transferred points you will have to wait until 9 a.m., when MS phone lines open. If you go the route of making two separate reservations and then linking the reservations, each of the two reservations will have its own 11 month or 7 month out reservation start date
Each use year will have its own banking date, its own annual budgets and statements provided, and dues will need to be paid separately. If you set up monthly dues payment via your checking account, each will be withdrawn separately.
I have two use years and have never had any problems as a result and the additional tracking and other work required is not that much more than having one use year.