It’s really one game split into two. There’s some new tracks in The Lost Age, but if you weren’t a fan of the first OST, the second isn’t any different.
The story continues in the sequel and rounds things out a bit more. If I had to pick between the two, The Lost Age is my favourite, but you can’t have that without the first.
Not saying you can’t play just one or the other, but they’re both made better by the existence of the other.