The mods could have the option to consolidate with each other. Each would have to agree to the move and could revoke that access at any time. If one instance goes down, the other still retains their posts. Both of them would still exist on their own, but anything made on one would automatically be published on the other.
This could have multiple levels.
At the most basic, posts are just automatically brought over between each community. The mods can take action that only affects their local instance.
You can then add an option to federate with the other community and any community they federate with. Mods are presented with options to disallow some of those communities if they choose.
Each community can then set moderation levels and permissions. There could be an option to retain moderation on federated posts made on the other instance for their local posts. IE if [email protected] removes a post made on [email protected], [email protected] can choose to have it removed there.