You cannot use a domain in multiple tenants. In your scenario, simply purchase "enterprise" licenses for your users, the 300 limit does not apply to those. If for some reason you do have to use multiple tenants, you can add a subdomain instead, i.e. have domain.com in tenant A, and us.domain.com in tenant B.
There is the "domain sharing" functionality that's currently in preview, but that's intended for use with tenant-to-tenant migrations and an overkill for your scenario. Here's the relevant info, just in case: https://learn.microsoft.com/en-us/microsoft-365/enterprise/cross-tenant-mailbox-migration?view=o365-worldwide