Two use years means two membership numbers and for all intents and purposes you should think of it as if you were two different people with separate memberships. The banking dates differ, you get two different annual maintennace bills, and if you have automatic withdrawal from checking account for dues, you get two separate withdrawals per month.
You cannot combine points from the two different use years to reserve any given night. Instead if you want to use them both for a single trip, you either need to first transfer points from one use year to the other, or, alternatively, make two separate, but sequential reservations for the same resort and room size, and then have MS link them so you do not have to change rooms at the turnover date. If you transfer points, you get a new issue in that transferred points do not show up in your online account and you cannot use transferred points to do a reservation at 8 a.m. online but instead must call MS at 9, and thus you lose the one hour on;line reservation advantage at the 11 and 7 month windows. If you make two separate, but sequential reservations, you also have issues you would otherwise not have with a single use year. Each of the separate reservations has its own 11 and 7 month window, and if dealing with a high demand room and season, such as VGF studios in the last quarter of the year, you could get the issue of being able to make the first reservation but then when the time comes to make the second one, you could be blocked out even at 11 months out because those studios can disappear before the 11 month window opens. Also, the 180 day out "plus-length-of-stay" rule for restaurant reservations applies separately to the two reservations.
An advantage to two separate use years if they are months apart, is that you can possibly avoid having the risk of not being able to bank points if you cancel a trip too late into the use year --if you want to take a trip in the last quarter of one use year, you can use the other use year to make the reservation and thus still be able to bank points if you cancel as the trip approaches.