The UI allows creating multiple CalDAV backends (and optionally giving them different, custom names), and they seem to work OK, but exiting and reopening GTG loses all but one backend of each kind.

The cause seems to be that the config file backends.conf has a single section per backend kind (i.e. one for backend_localfile and one for backend_caldav).

Maybe it would be enough to use backend IDs instead of backend names in backends.conf.