Hi Stuart,
Only one journal connector is required on the Mimecast side, and both the on-premises server and Office 365 can be configured to send journal messages to the same email address (e.g., journaling@journal.domainA.com
and journaling@journal.domainB.com
). This approach allows you to handle journaling for both domains without separating your Office 365 tenant or combining Mimecast entities.
To deploy instructions, you can get tutorials from Mimecast website.