My solution to this problem is to have only one notmuch instance 
run on a remote VPS and connect to it via "thin" notmuch-emacs 
front-ends, very similar to [1]. It has been working very well for 
the last few years.

[1] https://notmuchmail.org/remoteusage/

On Mon, 17 Dec 2018, Dan Čermák <dan.cermak@posteo.net> wrote:
> Hi list,
>
> first and foremost: thanks to everyone who contributed to notmuch, it's
> a truly awesome piece of software.
>
> Unfortunately, I am facing the problem (which I have been postponing to
> face for a while) that I'll need to keep my notmuch database in sync
> over multiple machines.
> Thus my question: how are you achieving that? How well does your
> solution work?
>
> (I have found muchsync, but unfortunately very little reports about how
> well it works, which isn't necessarily a bad thing. Beside that only
> some scripts around notmuch dump.)
>
>
> Thanks in advance,
>
> Dan
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