Might be able to set it up with an anonymous-reposter bot instead? Presumably this is for throwaway purposes more than confidentiality (the server owner will be able to track you either way) so having a bot (e.g. “@ThrowawayBot”) that you could private message and have them repost might work.
The major concern is whether to cross-post so that members of only one community can see posts from the other, or to avoid cross posts so that people subscribed to both don’t see duplicates