Welcome to the Auth0 Community!
I understand that you have questions regarding SSO between your applications.
Given that you have the first app using the Universal Login and the second app is using Embedded Login, and on different domains, this will not be possible. Having your applications on different domains mean that they cannot share the same SSO cookie. Therefore, it’s not possible to implement SSO across your applications.
With that said, I encourage you to consider migrating from Embedded Login to Universal Login or vice versa to accomplish SSO.
Please let me know if you have any further questions. I would be happy to assist.