2 Domains hosted, users with 2 emails

I have a test network setup with 2 domains being tested on a Exchange server. What I am trying to do is have each user or a couple of user each have to email address (@abc.com and @123.com) On the users machine I need them to have these 2 emails seperate. All that I seem to find and be able to do is have the second domain just forward to the main address. How can I split this up for each users. So they can check both emails seperatly?