Merging data between credentials

My application starts off by logging in each user with an arbitrary JWT containing a fresh UUID. The user can then log in with Google auth at any point in the experience (which links new Google Credentials to the user), or not log in at all and stay in this pseudo-anonymous state.

The use case I am considering is a user A that logs in with Google, and a user B that logs in with that same Google account on a different device. Right now I have the partition keys set up as using the initial arbitrary ID given in the initial JWT Credentials. What would the be best way to essentially merge realms for different credentials. For example, once Google Credentials are added, the realm for another user logged into those same credentials would be accessed.