I looked into both as well as YepNope, none of which did exactly what I wanted. So what did I do? I wrote my own.
https://github.com/sgarbesi/fallback.js
HeadJS didn't do fallbacks how I wanted it to and neither did LabJS or YepNope.
RequireJS on the other hand somewhat did but the library was way too large. The whole point of this library is for it too be as minimal as possible so the page can actually load as fast as possible.
This will allow you to load as many scripts as you want and have as many fallbacks as you want, it also gives you a function to execute your code in after all the libraries have finished loading.
Worth checking it out, in my opinion was well worth me writing it.