Letting Staff Work Across Sister Companies Without a Second Account

The challenge

Each company in the group ran its own directory and Microsoft 365 tenant, which is correct for separate legal entities and painful for the people who work across them. Shared documents, applications and channels each meant either switching accounts or holding a duplicate one — and duplicate accounts are the ones that never get closed.

Where we came in

The apparently simple fix is to merge the tenants, which trades a daily inconvenience for a permanent loss of separation between legal entities. Cross-tenant synchronisation gives the collaboration without the merger. Because identity changes are unforgiving, we built the whole thing in test tenants first.

What we did

• Created dedicated test tenants and proved the configuration there before touching production
• Linked the test environment through the partner and licensing platforms so licensing behaviour was tested too
• Configured synchronisation so users from one company are provisioned as guests in another rather than duplicated
• Validated against a real application before extending the pattern
• Kept each company's tenant separate, synchronising identity rather than consolidating it

Where it landed

• Staff reach resources in sister companies with their existing account
• Each company keeps its own tenant and administrative boundary
• The configuration was proven in test tenants before production, so the rollout carried no discovery risk