Afaik there is no software that allows you to sync with x servers and concurrently with x clients and keep everything in sync. You can only have one server for a “server wins” architecture. Ofc there is the possibility of mirroring or clustering on the server side but with 2 different types of servers (fruux and NC for instance) this would not be possible for various reasons.
DAVdroid doesn't work with citadel-server
This URL is not reachable and even if it was, I wouldn’t have user name and password.
Please provide steps to reproduce. Can you also provide a Citadel test account?
You can create an account on the project’s website http://uncensored.citadel.org
I can confirm this bug!
DroidDAV does not work with citadel
More information about citadel groupdav support:
An demo account can be created on uncensored.citadel.org.
It seems like Citadel doesn’t provide a CalDAV/CardDAV (which is not GroupDAV) service: » If and when CalDAV clients become generally available in a meaningful way, we will of course provide a server implementation of it. In the meantime, GroupDAV is available.«