First published on TECHNET on Nov 03, 2017 Hello All,
If you have the configuration as expressed in the article where you have two domains, and are migrating users without disabling or deleting the source account then you are in an unsupported configuration.
This means that you will still be able to open cases and work with our support engineers on any issues, however while they are working with you if they find that the issue leads to having those two accounts enabled then they will stop troubleshooting and you will be told to make changes to bring your farm back to a supported configuration (Delete or Disable source accounts).
The fact is that I have two customers who have been dealing with a configuration like this and it is a constant issue that has caused many calls to the helpdesk, I will Strongly encourage you NOT to do this and to find a way to disable that source account. This issue will affect Profiles, Workflows, Permissions, and anywhere that we use authorization or identity.