When you upload a new file, or edit a previously uploaded file, at the bottom there is a form for Related Projects, you can add the name of the other project there and once you click the "Add Project" button there will be a drop down to select the type of relationship, in this case sounds like "Required Library"
When you upload a new file, or edit a previously uploaded file, at the bottom there is a form for Related Projects, you can add the name of the other project there and once you click the "Add Project" button there will be a drop down to select the type of relationship, in this case sounds like "Required Library"