To add a new project into a solution, you simply right-click on the solution in the Solution Explorer window and select Add then New Project. You would then need to add a reference to it by right-clicking your controller project in the Solution Explorer window and selecting "Add Reference". Add a reference the new project. Finally, right-click your controller project one more time and select "Manage Imported Projects" and move your new project into the right-column. This will allow you to call automations and use resources from your new project in your controller. 

As a best practice make sure you add reference of all adapter projects to the controller project and import them.

Don't refer one adapter project to another. Use controller project to communicate between different adapter project.

This will make architecture simple. Down the line you will be able to debug issues easily.