They're SQLite databases so you'd need to merge each of their tables. This would take a bit of code, but should work.

Why not implement the corresponding option in mcfly?

How would you envision that option working?

Alternatively, you could try synchronizing the database using something like Dropbox or Syncthing?

How would you envision that option working?

Just an idea, no specific scenario.

Alternatively, you could try synchronizing the database using something like Dropbox or Syncthing?

Do you mean to use a centralized cloud storage workflow?

Now that I think about it, Dropbox wouldn't work because the database would always conflict. I'm not sure what a clean way would be to support streams from multiple machines besides having a merge tool, or maybe allowing it to read from multiple DBs that are then synced. I think this is out of scope for this project, though.

What about git + a merge driver like the one https://github.com/larkery/zsh-histdb uses?

I think this also would be a super useful feature, I've wasted multiple hours today trying to import another mcfly history.

I'm open to contributions of a merge tool, or an ENV variable that lists multiple DBs to read from.