If you plan to combine the points from smaller contracts for stays it will be far easier to keep them in the same UY. Different UY points will be under different memberships which cannot be combined “on the fly” for a booking. You would need to transfer the points first, then you can’t see the transferred points online, then you have to call MS to book at 0900 instead of 0800. Not an insurmountable problem, but you don’t want that process to be the norm IMHO.
With that said, we have two use years but only find ourselves in a transfer situation about every 4th year (by our estimate) for point balancing as we utilize the memberships separately which again IMHO (as suggested by many here) is the best practice generally for multiple UY utilization.