Adding a Second Domain Ability to Send and Receive? Is that Possi

I've been reading man threads regarding adding a second domain to SBS 2003,
and it seems very easy, but SBS 2008 is different due to Exchange 2007.

I have added a second domain via Accepted Domains.

I am able to receive emails fine from both Domains. But sending is a
different question, and is it even possible?

I have a client with two companies and they need to send emails from
different companies.

I read a few about setting up a security group and assigning a SEND AS, but
when I tried that, I receive this error;

"You are not allowed to send this message because you are trying to send on
behalf of another sender without permission to do so. Please verify that you
are sending on behalf of the correct sender, or ask your system administrator
to help you get the required permission."

I've tried to assign the Send on Behalf, and it still fails. (I removed the
Send As permission)

I've tried creating a new email user and I am able to sending using the
second domain via OWA, but when I try to setup the SEND AS or Send on Behalf
of, I receive the same error message above.

Any thoughts links on how to setup this up correct? Or is it even possible?