If you need to track users' behavior and interactions with website A and website B, you may consider Stape's Cookie reStore tag for the server Google Tag Manager.

This tag can store user identifiers and their cookies in Firebase and restore them when you need.

There are no restrictions on the type of user identifier (you can use as an identification email or user ID in the CRM, cookie, etc.)

You can check more info on the tag and how it works in this article: https://stape.io/blog/server-side-cross-domain-tracking-using-cookie-restore-tag