I think you mean to authenticate multiple users that belong to multiple tenants.

In order to do that you need to implement an extra table called TenantUsers. This will hold the mappings between the users stored in Duende and your tenants. Then you can store the connection string for each tenant in Azure Key Vault or something similar (depending on your cloud provider).
After login you can show a dropdown with a list of tenants and when the user clicks one of them then it connects to the correct connection string that belongs to it and displays the screen associated to that specific tenant.

For extra security you should also enable 2FA with Google/Microsoft authenticator apps for all your users.