Let's say we have two gpg primary keys "Ident-A" and "Ident-B". There is also a subkey "Sub1" used for encryption by "Ident-A". It is possible to add a subkey "Sub2" to "Ident-B" using the private key of "Sub1".
Now each primary key has a subkey with the same keygrip but different fingerprints. In addition to that, one can encrypt a file with the public key of "Sub1" with "Ident-A" and decrypt it with the private key of "Sub2" with "Ident-B" because both subkeys are based on the same private key.
To get this work, you have to use "gpg -R [UID] --encrypt file" so that the key used for encryption is hidden. Otherwise gpg will abort the decryption process because it cannot find the corresponding private key to the public key used for encryption although gpg has this private key in its keyring.
Is it possible to copy/clone a subkey of a primary key to another primary key without changing the fingerprint of the subkey? (only add bind signatures)