We have 9 contracts, all the same UY, spread over 4 home resorts. It’s honestly really easy. When we book at 11 months, we essentially treat the contracts as 1 pool of points using banked, current UY, and then borrowing (and for us it really doesn’t matter which contract is used 1st or for borrowing). It gets a little complicated at 7 months - we have a pool of SAP+ points that we use first (BLT) and then usually a few left overs here and there from other contracts. The booking system lets you select the contracts you want to use and in what order - it pulls banked first, then current, etc.
I did learn a good trick from the disboards on booking at 7 months - we needed about 1200 points for 8 nights in a bungalow. I wanted to use the BLT and any left over points from our other contracts but did not want to touch a specific number of banked/current/future BCV points (because we wanted to book 1 1br for 9 nights 11 months). A dis’er suggested I make a booking with the exact number of BCV points i wanted first to “shelter” those points. And THEN make the poly booking. After the bungalow was booked, I released the “dummy” BCV booking which then left me with the banked/current points I wanted/needed for that booking.
I know a lot of people have different UYs and manage them just fine. We very much have purchased contracts with the goal of keeping them all in 1 UY.