How do you open multiple blender files at the same time on a Mac? To allow me to copy and paste in between them?

Hey Richard, you could open multiple instances, which is most easily done through Terminal or by Right Clicking on Blender.app > Show Package Contents, then browse to Contents > MacOS and double check on the blender executable.

But the paste buffer still works after Blender is closed, so you could copy in one file, close it, open another, and then paste.