Select From multiple Database Connects for 3rd Party clients with the New Universal Login

We are in the process of migrating our users from multiple connections into a single connection. One of the connections is a Auth0 Database connection, which is a domain connection (is_domain_connection) because it is used by 3rd party applications. During the migration the users exist in both connections and because we need the refresh tokens of old users to still be valid until the users relogin and get a new refresh token from the new connection. We have very limited control over the 3rd party clients, so we cannot select the login connection via query parameters.

We are currently stuck, because we cannot enable is_domain_connection for the new Database connection, because then the New Universal Login cannot decide which connection to use to login the user. All Guides I found so far are about the Classic Universal Login with Lock which can be configured with js, but I did not found something similar for the new Universal Login.