That is an excellent question, Robert!
I’ve only seen one case where someone was able to share events between two sites on the same multisite network. It was actually a very impressive setup that allowed the sites to display the events from the main network site in the widgets of the other sites.
Unfortunately, that solution is no longer valid since the 3.10 update. You can see the original code for it here, in case that gives you a good starting point to work from to accomplish the type of sharing between sites you are looking for.
Sorry I don’t have more of a silver bullet solution for you here, but does this at least help answer your question? Please let me know.
Cheers!
Geoff