As to the original question:
Same use year means ease of keeping track and you can combine the points from the contracts to reserve any given night (at 11 months out if points are for same resort; at 7 if different resorts). You cannot do that with different use years. If you want to use two different use years for one trip in the same type room, you either have to transfer points from one use year to the other, or reserve some nights with one use year, and then reserve the remaining nights with the second use year, and that would be two separate reservations.
If you go the transfer method, the online system is still not set up to allow you to combine your points in one use year with the points in another use year to do a single reservation at 8 a.m.; instead you have to call MS at 9 to do that and thus lose that one hour difference in being able to get the reservation at 11 or 7 months out.
If you use the two separate reservations method, you can have MS link the reservations. Though there was some risk in the past, there is no longer any risk of having to change rooms or having to check-in twice when you do so. As long as you add both of the linked reservations to your
MDE account, the reservation system is set up now so that the resort will recognize those reservations as one continuous reservation and treat them the same as a single reservation at the resort.
The different use years will appear in your same DVC online account as long as the deeds have the same owner names, but each will show its activities separately (you click on one use year to get its information and the other to get its). They will be billed separately for annual dues, and you need to pay each separately. The banking dates will, of course, differ. Each could have different benefits restrictions depending on when purchased if they are resale contracts. However, for most benefits you just need one use year that qualifies for them (and make sure you bring the applicable card for that one when you go to WDW). Since the two use years will have different membership numbers, you can get the allowable two waitlists for each use year, and can purchase the allowed one time use points for each use year.