> I came accross the admin guide and I was reading / searching for a way to sync logbooks across sites...
> elogd mention "-m" and "-M" ... not elog but elogd... with that description :
> synchronize logbook(s) with remote server

If you would have followed the shown link to the "elogd.cfg syntax page", you would have found the chapter Mirroring:
https://midas.psi.ch/elog/config.html#mirroring

> Does it sync all logbooks ? 

Not necessarily, but that is the default.

> is there any examples somewhere or advice ?

See above: you'll find examples under #mirroring

> Thanks :)

You're welcome.