How to sync two CalDAV servers
Solution 1:
vdirsyncer https://github.com/untitaker/vdirsyncer by https://stackoverflow.com/users/1544347/markus-unterwaditzer can do this via CardDAV, two-way as well.
I'd advise doing it via multiple runs against a local "vdir" rather than directly, and you can even implement git VCS for data security.