After an acquisition, the default assumption is that the acquired business moves into the parent's tenant. Sometimes that is right. Sometimes keeping two tenants that collaborate well is cheaper, faster and less disruptive.
What cross-tenant collaboration gives you
Microsoft provides cross-tenant access settings, cross-tenant synchronisation and multitenant organisation features. Together they let people in both tenants find each other in the address list, book meetings with free/busy, chat and meet in Teams, and access shared sites with their own accounts.
Reasons to consolidate
The businesses are fully integrating, with shared teams, processes and brand. You want one security baseline and one place to manage it. Licence consolidation will save money. Regulatory or customer obligations need one controlled environment. Or the acquired tenant is poorly configured and fixing it costs as much as moving.
Reasons to stay separate
The acquired business will keep operating independently. It may be sold again, which makes a later split expensive if it has been merged. It operates under different regulatory obligations. Or the timeline and budget don't support a migration yet.
The middle ground
Many integrations start with collaboration and consolidate later. That is a sound plan, provided the acquired tenant is brought up to your security baseline while it stays separate, and the consolidation decision has a date rather than drifting.
Security either way
Collaboration extends trust between tenants. Before switching it on, check the other tenant's admin accounts, MFA, Conditional Access and signs of compromise. And manage the acquired tenant to the same standard as your own, whether or not it is merged.
Decide on purpose
Write down the reasons for the decision, the conditions that would change it and the date it will be reviewed. That keeps the question from being reopened every quarter.
Planning a tenant migration? Tenant-to-tenant migration
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