We have a system with just 1 domain & 1 po

We now need to add another company - this is to exist a seperate company

I've created a OU for the new company, created a PO and and users
I assigned their internet domain to the PO ie. newcompany.co.uk on the PO
internet addressing entry and set the flag "for incoming mail recepients are
exclusively know by this internet domain name"

I'm now told the company also has another domain newcompany.com

How do I set this up - we don't want users to be able receive emails as
only as user@newcompany.co.uk or user@newcompany.com

How do I set this up ?