OK, I just tried it...
You CAN copy and paste...
but not everything
It would be nice to right click a layout and choose 'Copy' and paste in the other project, in this situation you can select everything in the layout copy, then paste it in the new project. What does NOT get copied is anything that may be invisible.
I can confirm that families don't paste, and possible other sound/music etc bits.
Then for events, you open the event sheet, select it all, then copy and paste.
(It the object doesn't exist the paste will fail).
So its not all encompassing, but its a bit,,