I believe this is a frequently asked question. Here's my understanding of the answer.
To use one library from another, it is necessary to include both libraries from the sketch, and make any references to the libraries from sketch-based code. (This can be awkward…) The Bitlash examples that include other libraries are structured this way: bitlash/examples at master · billroy/bitlash · GitHub and yes it can lead to ugly things like passing around function pointers.
I've had no luck including one library directly in another. I hope someone will correct my understanding if it's wrong.
-br