You can have federation between domains or trusts but not both.
If you have 2 top level domains with no trust like contoso.com and fabrikam.com they can each have an ADFS Server and be federated to each other. But if the 2 domains have a 2- way trust then only 1 ADFS Should be used as it is a client to AD and would use UPN suffix routing just like any other AD client would.
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