Nicolas,
as described
here in French, I confirm there is no such DNS synchro aside the Samba related part
Thus DNS high availability thanks to this DNS synchro is not feasible (using default Zentyal features).
Same if you want to reach hosts described in DNS for 2 different networks linked via 2 Zentyal servers, you have to maintain your zones aligned manually or configure remote DNS in you DNS list.
This looks simple but as each DNS thinks to be authoritative for the domain... this should even not work... to be tested