Hi Meggie,
Thanks for the additional information there, there is definitely demand for and a solid use-case for being able to share licenses and even projects cross sites, but unfortunately we just don’t have the infrastructure in place to be able to do this at the moment.
Sadly the only way right now would be to use 1 Shotgun site as the main site, and distribute licenses to access that site. Like I mentioned above we have seen studios setup ways to sync or copy date from a project between sites.
I’m really sorry we don’t have a better solution as of right now, it would be great if you wouldn’t mind adding your support and use-case to our roadmap (instructions on how to do that here)
Have a splendid day! 
Cheers,
Andrew