Splitting a 365 Tenancy?

Thanks for your response.
No decisions have been made yet, I'm investigating the options with a view to both the convenience of the users and the separation of information. As it happens it seems from my initial enquiries that some of the perceived issues may actually be coming more from poor communication between admins and users rather than limitations of the M365 setup, and for the others there's probably more than one way to skin a cat, for example information that should not be visible across branches could simply be kept off M365.
That being said, my job at this point is to identify the options and their plus and minus points
Where can I find some information on the limitations of cross-tenant collaboration?
As for domain sharing, I think (subject to some discussion) the only thing that matters in this situation from a user perspective is email addresses (as they are already established for external communications), and that should normally just be a case of routing things to the right places, or does 365 throw a spanner in the works in some way?