Email registration across different titles

Hello, we are working on our second mobile title using Playfab, in which players first login using their iOS or Android DeviceId, and later register their account using email and password. The problem we anticipate is with players that had already registered an account in our first game, when they try to register their account on the second game and they can't because that email is already 'taken' (since player accounts are shared across all publisher titles, and email accounts are unique).

Of course, it would be ideal if the player was signed in straight away into their existing account from the first game, and didn't need to register again in the new game. That's unlikely to happen in most cases, though, as we cannot ask them to log in with their existing account from the first game (we technically could, but it's off the table for commercial reasons) and we can't trust their DeviceId to be the same one as the one linked to their existing account.

Has anyone faced this problem before? Any suggestion on how to best approach it? The only reliable solution we can think of is to create a new Publisher in the Playfab dashboard and house the new title there, but we imagine there has to be a better way to do it.

Thank you in advance!