Providing that you purchase the same UY and title it the same way, what you are really doing is adding on to your membership. The contracts themselves are individual. If you add on through Disney, then it will automatically be the same UY.
Nw, if you buy a different UY, or with different names, then you are really not doing an add on but purchasing a new membership and you will have two membership numbers.
We just did this. It wasn't my first choice but the couple of contracts that were in my same UY were more than I wanted to spend. Of course, I got my DH to agree to let me get a bigger contract to compensate for the different UY so it will work out because those points are for a specific purpose.