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How does that work? Do I just add the other community ID as a tag and it will override the default hive-147010?

Posting through liketu.com makes the liketu community the first and default tag. You can then tag the other community ids through the interface and the post will also appear in other communities.

mmm.... is that treated as two separate posts on Hive, or a cross post from the liketu community to the other community?

It's a single post which is just aggregated on different communities.