The borrowing rule does not come into play because you can not borrow points once transferred. That is why you have to be strategic if you are going to do it when having multiple memberships...which, to be honest, I have done only a few times, and won't again. I'd rather just book some nights on one and some on the other and have them noted so we don't have to change rooms...which we never have.
For example, I have an Aug UY and Dec UY membership. If I wanted to travel in September 2023, it takes place in my Aug 2023 UY, but my Dec 2022 UY. I can't transfer my Dec 2023 UY points to my Aug UY and have them eligible for that trip because it would require "borrowing them" since no matter what UY they are sitting in, they remain Dec UY points.