There is no UI for this usecase.

You should be able to modify the underlying configuration files yourself though, and as long as you don't save your configuration via kube again this should work (haven't tried it though).

You can find the configuration files by:

• Finding the configuration file location using "sinksh info" under Config location
• Find your resource id using "sinksh list resource" (use server and type to find it's identifier)
• Edit the corresponding configuration file in the config location, e.g. ~/.var/app/com.kubeproject.kube/config/sink/\{887f4e0b-9cc1-4f4e-8e1b-4e0294deeae3\}.ini (in flatpak).

For a commandline in the flatpak use:

$ flatpak run --command=/bin/bash com.kubeproject.kube

Out of interest, would this require the same password to be used for the CardDAV and CalDAV servers as well?

The kube codebase currently indeed makes that assumption and sets the same password on all sink resources of the same account during regular usage.

There is a workaround for that as well though. If you start kube with a --keyring argument which takes a json object you can set different passwords for different backends as well.
This will result in the login screen never showing up because you will automatically be logged in:

kube --keyring '{"$accountId": {"$resource1Id": "'"$PASS1"'", "$resource2Id": "'"$PASS2"'"}}'

The quoting in bash get's a bit funky, but I hope you get the idea.