Tell me more about this single login. When you go to make a reservation, do you have to select which "membership" (UY) to use before you search for availability or is it the same as having a single UY where you simply choose which contract(s) to use (and in your case have choice of multiple contacts from multiple use years)? If the latter, seems that would solve the multiple-UY problem since you aren't transferring points, you are just selecting multiple contracts at time of reservation. Just curious how multiple UYs actually works
in the member site with one login, since despite this being a pretty common scenario no one has posted how this works online

, and all the historical threads, FAQs, and stickies all are antiquated (pre-date the current member services website with single Disney Account).