Thanks but I think that this is not the solution to my problem: (
I think I have omitted some important information:
1. The two servers are not master / slave but a standalone
2. each server has its DNS services, mail, web ect ...
3. I think the sync tools you suggested are not suitable to my problem, most are not even present in the ispconfig installation directory on the servers.
ispconfig version is 18.104.22.168
many thanks for your patience: D