Thank you for your question on WorkMail. For your use case, having two domain in a single organization is possible. You will configure the domain per user. this means that the users in your organization do not get both domains automatically. The isers within the organization can see other users from different domains in the address book.
Since you only pay for enabled users there is no cost difference in having all users in 1 organization of spreading them over several organizations.
I will also ping the service team to see if this question is in the WorkMail documentation and ask them to review it for clarity.
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