I would like to add a project to an existing project.

I am trying to add a photo project to my existing project. Both projects work independantly. I did the the file add procedure and now both projects work independently when chosen from the project. I would like to link them up now by adding some navigation between them, Maybe a button from one to the other so I can access the function and come back to the original. I cannot get an access to link the two. I am wondering is there a way to do this.

Project A gets a button that connects with Project B then Project B gets a button that takes you home to Project A.