I have a technical question.
A wallet is protected by a password, which encrypts the key.
Is it possible to "merge" two encrypted wallets into one and have a completely new database, if they are encrypted with the same password?
Is there any paper on how the encryption works?
First you would need to know if the DB is same version, otherwise you would need to convert it yourself.
This is possible in blockchain.com wallets (to have multiple xpriv) but as someone said above, it would probably mean you would break backward compatibility and would have your own custom bitcoin core version not readable by other versions.