Yes, the rule is that you cannot merge two reservations made with points from two different use years (which would mean you have two separate memberships). That is even the rule if the points are for the same resort and the reservations are made at 11-months out, although in that situation you could first transfer points from one membership to the other and make a single reservation at 11-months out.
In the last 16 years, I have actually done a number of consecutive reservations using different use years. In that situation we contacted MS and requested that the reservations be linked. We have never had to change rooms, although to help assure that, when we arrived at the resort, we went to the front desk to make sure it was aware that we had two linked reservations.