[Parody]

A local .realm file (and any backups you make) includes your login and all of your downloaded realms. You can't separate one realm from it, though you could delete all the other local realm data first, I suppose. To restore someone else's realm you'd need to delete your current master.realm file (or use a new Windows account) and log in as them. I don't know what happens if you delete everything in a local Realm Works master folder and put someone else's master.realm file in it, but my guess is that you'll get an error due to mismatching computer IDs.

I can think of some things you could try despite the above, but the interaction between realms and accounts risks losing control of the account involved and software piracy, neither of which are good things.

It would be nice if they had designed it so each realm was in its own file and you could work with them like any other file, but that's not how it works. :(