Two companies sharing one network. How to setup domain and email?

The but that confuses me is what I need to do so that users from company1 send from their domain name by default and those from company2 send from that domain name by default.
How do you get around the fact that SBS08 can only have 1 primary domain name?

This is all making sense. Thanks for the help so far.
Ultimatley I will be doing this on SBS 2008. However Im currently testing it on SBS2003 as its what I have available....
How do I change the primary address for a user that they send and receive from company2.com?

Perfect. Found it.
Seems to be working well. Quite simple really. All the how to's and posts on the web make it sound more complicated then it is.
This is what I have done on the working sbs03 server to add a new email domain;
1) go into the default recipient policy and add a new SMTP address.
2) create a new user account in AD.
3) go to the properties of new user > exchange general tab> click on the company2.com smtp address and set to primary.
4) remove the company1.com smtp address from the users properties as well.
When I set this up on the new SBS 08 server I will be creating a new AD domain at the same time. What I will do is create the AD domain using a name seperate to the two companies. eg: I may use the building name.
Final question; is it possible to setup a user template for each company. This would make it easier when creating new users as you wouldnt have to go manualy edit the users primary email domain? Or is there another shortcut?
Thanks everyone