If you get the same use year and inform Disney before closing of that fact, the contracts will be under the same member number and can be mixed together to reserve any room for any given night at 7 months out, and everything will be just like having your original contract, including banking date, except that you cannot mix points from two different resorts to reserve one at 11 months out.
However, if you purchase a different use year, your world will be substantially different. You will get a new member number for that second use year and for most purposes it is like being two entirely different members. As long as it is deeded to the exact same owner names as your original use year, the different member numbers and use years will show up in the same online account in the
DVC member site but as separate member ownerships.
Each use year will have its own banking date. Each use year will have a separate annual bill and each needs to be paid separately. If you have automatic withdrawal from a checking account for dues, there will be two separate withdrawals per month.
The points from one use year cannot be mixed with the points from another use year when making a reservation. To be able to use them together at 7 months out, you will either have to first transfer points from one member number and use year to the other, or instead do separate sequential reservations: (a) you book some nights of the trip with one use year, (b) then book the other nights, with a different arrival date, and thus different 7 month out date, for the remaining dates with the other use year, and (c) then call or email MS and inform it of the two reservations and separate member numbers and have the two reservations linked so you do not have to change rooms.