Having two different use years requires a little more tracking but it does not otherwise have significant impact. The differences:
1. The two contracts are separate. Each will have its own banking deadline that applies to the particular points in the contract. Thus, you need to keep close track of both contracts.
2. You cannot just mix them at 7 months out to reserve any particular night in a trip. Instead, to use both for a trip, you need to use points from one contract for some of the nights, points from the other for the rest, and then have the two reservations linked so it becomes one reservation (you, of course, can do that all in one call).
3. You pay your dues separately on each. Thus, if you have automatic monthly withdrawal on your checking account, you will have two separate withdrawals per month rather than one.