Buying another contract will not change your status for member perks. In terms of UY, I have 3. Yes, it takes a little practice to get used to but I have not yet found it that difficult to manage. However, each UY I own has enough points that they function well on their own and are used for different trips throughout the year. Depending on how you want to use it, will determine how easy/hard it is. Is there a reason you want a new UY? It is certainly easier with the same one, especially when you want to use both contracts for the same trip. In this case, you have to book whole nights only for each contract and then ask MS to note on each one that it is a continuous reservation so that you can stay in the same room. Or, you have to use the transfer option and then call MS to book under one of the memberships. The hardest part is that there could be a time where you need to borrow off one contract when you have current UY points in the other.
For me, because I bought for different purposes, and so that when I went, I was always within the first 8 months of my UY, I was willing to manage 3. I do need to constantly keep track of banking deadlines and things like that, but so far, its been okay, and I have been okay with borrowing when I needed to to book what I want. Having said all of that, if you are thinking of adding on with a small amount, I would go with the same UY. Good luck!