I had a similar problem, with a couple different projects that people were often switching back and forth between. I probably would have developed them as parts of the same project, but it was like that when I started working here, and it would take significant effort to combine them, and people were already used to the navigation structure of each project.
What I ended up doing was creating a "wrapper" project that creates a side menu navigation between the projects that toggles their display within Inline Frames. Whenever a user logs in under one of the projects, a database entry logs their host and the time. Each project invokes a script every few seconds when not authenticated to see if that host logged in within the last few seconds, and if it did, invokes system.perspective.login() which automatically logs them into that project.
Not terribly elegant, but it works, and creates the illusion of a single project.