Two Domains On One GW6.5 Server

I want to have two external domains on the same server. Very few users and very little email.

I have a "mydomain.com" now, and I want to set up another domain for a non profit to use on my server. I want them to be able to access it with web access or the GWClient if we can do that. although I have read tons of tids, I have not gotten a clear understanding if I can actually do what I want. When I have tried, I have had difficulty getting the links set up between the new domain, post office. What all do I need to do? What second GW components do I need?

I can set up another box and run another server if I have to but I would rather not. Just not enough volume or users.

Kind Regards!

David