Hi Jens,
thanks for the information and feedback.
Good to know that I was on the right track.
After posting this question and some hours of playing around with Sync Gateway and Couchbase Lite, I had a working Implementation, of course a very basic one.
Anyway, the hole time I found the solutions to complicate for such a simple operation and I reconsidered my requirements.
Sync a Xml file from one machine to the other with some sort of authentication and failover.
I realized, that’s obviously Dropbox 
Of course we can’t use Dropbox, but there are other solutions out there like OwnCloud or Bittorrent Sync which fulfill all requirements, sync, authentication and failover.
So, again, thanks for your advise and if we need some sort of server -> client communication I come back to Sync Gateway.
Greetings
Marc