Under “Project Settings” and “Project Shortcuts” you can create shortcuts to other projects.
Just select the projects you would like to link together there. Make sure you click "Apply" at the left bottom before you hit "Save".
You need to have the same user in both projects, both of them needs to be distributed and both projects needs to have the other project selected in Project Shortcuts. If this is the case, the user can switch between projects on via the top left "Select Project", see the picture below.
Note: Project shortcuts will give you a drop down list in the right corner of project, where you easily can go to other projects that you want your users to see. However, if can look different depending on the template you are using.
The links or project shortcuts will not show when you are in the project in admin view. It will only show for users and if you look at the project from the list of project on My Page.