Adding 2 different companies as accounts

Adding 2 different companies as accounts

In one business unit, we have 2 accounts. Account1 and Account2.
 
Account2 is a branch of Account1. They have same domain but getting different support packages from us. IT and Office. What is the best practise of adding these accounts?
 
When adding separte accounts I cannot add the same domain name to them. So when sending request, one of them wont associate with their account and contract?
 
Regards.

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