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You have not said if you are a Cloud or a Premise Customer.
If you are a Premise customer you can specify alternate URL's to access the tenant on the Alternate Login URL on the Tenant Record in the ConfigDB:
So if you have multiple customers each with their own URL you can add their URL here. You just then need to make sure that the email that goes out to the person with the link in it has the correct URL and not the generic URL.