How do I open only one library in iMovie? I'm creating different libraries for each of my projects. When I attempt to open a new library via File > Open Library, it has both the old library open and the new one open.

I want to close the old one so I don't accidentally create projects within the old library. 

File > Close Library is disabled. If I try to close iMovie and re-open it, it remembers the last library that was opened.

How do I open just a new iMovie Library?
Find the Library in finder (e.g. ~/Movies), and double click it.

You can be sure that it's the only open Library by using the drop down menu in Projects: