In your organization, are they the same individual? If not, how do they work together and what kind of developer tools do they use?
I cannot really answer your question but still want to try to help via a slightly different scenario. In our team's case we have server side developers and mobile developers. We collaborate in such a way that the mobile team requests API's and the server team provides them if they make sense for the server or if they do not already exist.
Swagger is a good format for specifying API's. If the API is defined in Swagger first, the client side can implement against a Mock API which reflects the final server side implementation.
I hope this helps!