Yes, it's possible - to do exactly what you want. And it's easy, too!
Check out:
http://support.microsoft.com/default.aspx?scid=kb;en-us;268838&Product=sbserv2003

If your two external email domains are (abc.com and xyz.com), and you create
a new user 'Sally', she would be set up with email links for sally@abc.com
and sally@xyz.com. Within AD properties, you can identify which one is the
primary email address.

Note, all you are doing is identifying a second email domain for email.
There is NO need to worry about your SBS domain name (that is you DON'T have
to try to set up abc.local and  xyz.local).

Kal
Nice of you to want to share the resources, but hopefully you understand all
the issues that go along with that when this is a totally different company.

You will have to make each of their users members of your SBS AD domain, you
will need CALs for each of them, etc.   Any folder which has Everybody or
Authenticated Users access will be accessible to them as well.

If you set them up as just pop3 mail clients, then you have to change the
default settings on your Exchange Server to give them the ability to send
mail through Exchange and you run the risk then of exposing your Exchange
Server to becoming an open relay server

Just want to make sure you are aware of the security issues as well