You are unable to add members to a secondary instance using a secondary login. You receive an error that the email is already in use.
When you try to do invite the user to the second instance using Universal ID, you will get an error stating that the email is already in use. To resolve this, you will need to delete the existing login for the user in the second instance (login2.user@domain.com) and instead invite their primary login (login.user@domain.com) to the instance.
This will allow the user to access both instances from a single login
For more information on Universal ID: Using a Universal ID for Subscription Login - Marketo Docs - Product Docs
Who This Solution Applies To
Subscriptions with Universal ID active