Answer
No — on Apps10, a single user account can't be configured to log in via both SSO and Database Authentication simultaneously. Database Authentication specifically requires the user's Identity and Directory ID values to be exactly identical, or the login fails with a token verification error. The ALLOW_DB_AUTHENTICATION setting is associated specifically with an Apps9-compatibility mode carried over into Apps10, rather than a way to genuinely offer both authentication methods to the same user account at once.