We've got two contracts with the same UY, as well. In general, it makes everything easier to manage. However, I did have one problem once explaining to MS that all my points needed to come out of one of my particular contracts since I needed the other contract's points for the 11 month window when a waitlist came through and took points from the wrong contract. She kept telling me, "You're inside the 5 month window, all your points are equivalent now for the trip you're planning." I kept saying, "But I need all my BWV points for the trip I'm going to book as soon as my 11 month window opens up. This trip needs to come out of the AKV points." I thought we had it straight, but when I checked my account, the website showed the points about half and half from each contract. I had to call back, deal with a CM who refused to make any changes, and finally talk to a manager. I did get it straightened out, but it took multiple calls and was a huge hassle. If the contracts had not been same UY, this could not have happened.
So, while for the most part it is easier to have everything under one UY, it can create problems sometimes. Assuming we do eventually add-on more points (which we may or may not do; we want a Hawaii timeshare, and we're waiting to see what happens with Disney Ko'Olina before we buy one,) I'm not going to be absolutely determined to get the same UY. Maybe my experience was a fluke, but what it taught me is that there are pluses and minuses to having everything under one contract.