According to
Onyx Boox support on Reddit they can't do it.
The simplest way that I can think of is to use Koreader on all devices. It has a feature called
progress sync that does just that.
Onyx saves all info in a database, so that means either extracting and syncing and importing into the databases, with the hassle of checking for latest info..,
Yes, it could work with a small script that maintains a file with the info and timestamp and syncronizes it with the database and for instance Syncthing to synchronize those files over the internet. So you would be setting up three(!) sync's. Looks like a perfect way to introduce errors to me.
If you don't like Koreader, I saw other apps being thrown into the mix on Reddit, a.o. MoonReader, ReadEra.