Carcassonne: The River II playable/worth it without River I?
The River II does not require the River I in any way. I don't believe there is any official way to play with both of them together. IMHO The River I is a poor expansion by itself, in my play experience with it whoever goes first plays a farmer and whole shoreline on both sides of the river is one big connected farm giving that player a strong advantage. The River II fixes this issue by having roads and a city that cross the river, as well as one lake abutting a city, giving strong potential for multiple farms. All-in-all, River II is a good replacement for River I.

I've adopted some house rules that someone on here (maybe @Aramis?) suggested in a comment for using both River I and II: make river of random length by starting with a spring and not separating out the lakes and other spring tiles. The river is not allowed to have 2 springs without lakes, but keeps going until someone draws a lake to end it. This would produce very short rivers with just one expansion, but would work equally well with 2 copies of River II as with I and II.