I've not heard of there being two UTs, but the title
Unfinished tales of Númenor and Middle-Earth is sometimes interchanged with the shorter title
Unfinished Tales. They are the same book, you'll be glad to know

.
However, if you are buying HoME, the first two books of the series are entitled
The Book of Lost Tales Volume 1 and
The Book of Lost Tales Volume 2, so when it comes to Lost Tales, there are indeed two different books.
If you're short of cash, why not start collecting the books in the series with those that most spark your interest? There's no reason why you have to read them in order. Look
here for more info on each of the books and decide what you most want to read about!