Dual develop branches-automated merging

[Dustin Platter]

     The title is fuzzy so let me better explain. We have a project that is an MVC library. The Develop branch contains just the MVC library, but obviously contains no executable front end. We have another branch 'Develop_w_FrontEnd' that does contain a small front end interface using the MVC library.
     What I am trying to do, and hope someone can learn me, is this: We currently branch off Develop and create pull request back to Develop, but how can i have this automatically merge back to Develop_w_FrontEnd as well?
     Currently we make a 2nd pull request, 1 for Develop the other for Develop_w_FrontEnd. I'd like to automate that 2nd part in order to clean up the Pull Request tab. If it's approved and merged to Develop, then it can automatically be approved and merged to Develop_w_FrontEnd.

Can someone explain how to do this?