Web dev's dilemma: integration resopsibility

Situational question: AJAX POST to /user/create

Frontend HTML dev should find out URI /user/create and put inside JS, or;
Backend dev should put that in after completing user controller?

This grey area responsibility keeps popping up during the day
In some complex application with AJAX GET, POST, Facebook API with multiple end point URI
the communication effort is near to the development effort itself

Frontend dev do not want to dig inside controller and methods
Backend dev do not want to look at jQuery objects, success callbacks and even the HTML handling
However, in between this dilemma, there is still someone need to take this responsibility

We need an effective and somehow fair method
Or this is the essential communication between engineers?
So how remote teams handle these situations?