FWIW we just got a contract in process for what will now be our 2nd UY (and 4th home resort); and for our specific purposes I don't think it'll be much of a headache.
We usually book split stays so I'm already using 2 different contracts at the 11 month window anyway. And I rarely ever foresee that we'd need to combine points at 7 months.
It's also a very real possibility we could be looking to sell off something within the next 5 years, so the ability to transfer points into another membership and then sell the contract stripped is a big plus in my eyes. Having another membership also gives you the ability to have 2 more active room waitlist requests, which could be quite useful to have. So there can definitely be some positives with a 2nd UY.
But yeah as others have stated if you stick to mostly using them for separate trips or for split stays I don't see a huge issue, but if you regularly plan on combining points then I would reconsider. And I definitely would not have a direct contract split into 2 different UYs (which is essentially the same as having 2 different contracts at the same resort in 2 different UYs) as I feel that would truly be unnecessarily overcomplicating everything. That's just my $.02 anyway.