Easiest is one UY, but a lot can depend on what you are considering for your add on and why. If you are going for a different resort and essentially intend to use it separately then it's less important. If you are adding on at the resort you already own it will simplify if it's the same UY but it wouldn't be the end of the world if you found a great deal in a different one and bought that. If you think you will often want to use both contracts at 7 months, then a single UY will simplify things a lot.
Also, if it's a small add on you are thinking about then I'd try and stick with the same UY. If it's large enough to essentially stand on it's own for use then not quite so important. There are a couple of benefits of having a different UY if it makes sense for your use. For example if you travel regularly at different times of the year you can use whichever contract gives you the most time to rearrange plans should you have a last minute cancellation. With multiple UY's you also get 2 waitlists for each UY. But, you'll also have 2 banking deadlines and it can get a little complicated if you want to use them both at 7 months.