So I can speak on this since I own multiple use years, with 3 contracts at Riviera, 1 at AUL and 1 at AKL. If you have multiple use years, you will have two MEMBERSHIPS, but you will have 1 single account to login to the website. When booking, there is a dropdown menu that allows you to select which membership you are using but I can see the breakdown of all my points under one login account. Both of my use years have a healthy amount of points in them so I’ve never had to transfer points but have had to book thing separately because I do own multiple use years/memberships.
In general, I recommend against multiple use years unless you have a specific use for owning multiple. If you want to keep things nice and simple, stick to one use year. If you have things under one use year/membership, it will allow you to book everything as one big reservation.
edit: Small inconvenience but as you can see on my dashboard my Boardwalk BW view studio is separated into 2 parts and that's because when you book things with different memberships, it prevents you from "merging" reservations even though everything shows up under one dashboard. You can ask member services to "link" the reservations so you don’t have to move rooms and that request is typically honored. Also yay under 2 week mark to next WDW vacation
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