1. Add two projects to a session, for example "active" and "current"
2. Make the "active" project active (so Ctrl+R executes that project, for example). It will be marked in bold lettering in the projects view.
3. Select a file in the "current" project to make it current.

Now, pressing Ctrl+B will build the active project instead of the current project. In the Build menu, there will be items called

Clean Project "current"
Build Project "current"

Yet selecting any of these will build/clean the "active" project instead.