Here is the main part of original report to gnupg-users:
I want to setup one backup key as an ADSK for multiple keys. After
adding the ADSK to the first key, further attempts to add the same
ADSK to other keys fail with the error message:
gpg: key "44883766ABE65F20453E6FC046D03490A60D7131" not found: Wrong key usage gpg: Did you specify the fingerprint of a subkey?
My guess is that the fingerprint is resolved to the ADSK of the first
key with key usage R instead of the original subkey with key usage
SEAR. If I delete the key with the first ADSK and try to add the ADSK
to a second key, gpg can no longer find the original subkey:
gpg: key "44883766ABE65F20453E6FC046D03490A60D7131" not found: No public key
How can I configure the same subkey as an ADSK for multiple other keys?