Can you add more than one iMovie Theater account to an Apple TV?
As of June 2019, iMovie Theater is no longer supported and people are being moved to using iCloud Photos instead.

That being said, it appears that there is no way to add an additional account to the 3rd generation Apple TV, only the one iCloud account is allowed and thus only one iMovie Theater account can be used at a time. If you want to switch, you need to log out/in with the other account.