Yeah, I have one domain "anotherdomain.com" which used to be configured to mirror my main domain "domain.com" but now is set to redirect to it. I also have a two more domains on there as a courtesy for other people which are their own sites, though one seems to have expired now as they didn't renew it.
I also have several subdomains, for myself and others, which if I wanted I could use to host totally separate sites, but I just use mine as an easy way to send people moderately large files usually.
My host has a unique web panel, so simply telling you how I set it up on mine won't help you with other hosts. However, since you have presumably already added the first domain to your account, if your host allows multiple domains, then in theory you should go about it the same way when adding your second domain.
If you can't figure out how to do that, then fire off an email to their support and they should tell you exactly how to do it (assuming it can be done).