Tenant to Tenant migration

Hi,

our company is planning to do a split to two companies. We are considering migration of ~30 users to another M365 tenant, and their custom domain as well.

When I wanted to add that custom domain to 2nd M365 tenant got this error:

We have confirmed that you own custom_domain, but we can't add it to your account because the domain is already added to a different Microsoft 365 organization: ourmaintenant.onmicrosoft.com.

Removing that domain prior the migration should break the mail flow. Need advice how to proceed here.