You mean so that domain1.com and domain2.org both show the exact same site? That's called forwarding and is set up at the registrar or DNS side of the domain.
If you're talking about adding another domain to your account so you can host 2 from one hosting account, it depends on your service. If your host will not allow you to have more than 1 domain associated with your account, then you can't do it. If your host allows more than one, you should read their FAQ as I'm sure it's in there somewhere.
I know Servage will allow you to run multiple sites in the same hosting space.