How to setup Pycharm for multiple projects

The accepted answer works and was probably the only way to achieve opening multiple projects when the answer was authored. However, adding a new content root simply adds the files in the underlying directory to the open project. Project settings such as the Python interpreter have to be shared between content root folders.

At the time of writing, PyCharm genuinely supports multiple projects in a single window. This allows different projects to be configured differently.

The official help document contains a step-by-step guide with screenshots on how to open multiple projects in a single PyCharm window.

You can add directories to one, for example. Let's say you have ~/src/py_project1 open as a project. go to settings -> project: -> project structure then +Add Content Root, and add the directories one at a time.