Recently I worked with a partner that was doing a larger tenant to tenant migration. Fortunately they were moving from one domain to another and are doing a staged migration with MigrationWiz; because they will have users in one of two tenants, they will need to share free/busy information between the two tenants. This is easily done by setting up Exchange Organizational Sharing between the two Office 365 tenants.
Open Office 365 admin center
Click Admin Centers
Click New on Organization Sharing
In the Relationship name, enter a friendly name for the relationship
In the Domains to share with box, enter the external Office 365 or Exchange on-premise domain
Click to Enable calendar free/busy information sharing
Set the sharing level for calendar free/busy information
Set which users can share calendar free/busy information.