Two UYs are different memberships and the only way to use points from both is a transfer. Without it, you can book some nights on one and some on the other,
That is the only options you will have from this point forward. I always book some on one and some on the other and don’t combine.
So, I had an August trip booked this year that had 4 nights booked using August 2020 UY points, and then another 2 nights booked using my borrowed Dec 2020 UY points..which put them into my Dec 2019 UY.
I simply called and had them link so we didn’t have to change rooms. Of course, we ended up not going, but I never transfer,
So, if you wanted to go in Feb 2021, you can book a few nights in your Feb UY membership...and then borrow points from April 2021 UY for the other nights,
Of course, that is dangerous as if you cancel, they’d be stuck as they’d expire.