A different UY means a different membership. As mentioned, that means that you can not combine the points for a single nights reservation without transferring between the two.
There is a limit of one transfer, in or out of your membership per UY...while MS has allowed some leeway with this in terms of the same owners, there is no guarantee that will happen.
I own two UY's and have not yet found it to be a problem but that is because I bought 150 points when I did it. I think that helps me to keep them and use them as their own membership. At one point I owned only 50 points in a different UY and I found that would have proved difficult so I sold.
Good luck!