Merlin:
Yes, it is possible, and there are many ways to do it. From the SEO point of view though (to avoid duplicated content penalties) there is only one legit way to do it, and it's this:
Set up your second domain as an add-on domain within your existing account. Some hosts charge a small monthly fee for add-on domains, some others don't (hopefully yours doesn't

). You can read this article to learn how to set up an addon domain:
http://www.accordmarketing.com/tid/a...n-domains.html ).
When you set up your second domain as add-on, it will basically create a subdomain called seconddomain.maindomain.com . Then, go to the Sub-Domain section of your control panel and redirect your recently created subdomain to the folder within your existing account where you want your second domain to point. In other words, redirect seconddomain.maindomain.com to maindomain.com/folder/. And, you're set.
Now, if your second domain is not registered with your web host, go will first have to go to your domain registrar's page and change the DNS servers of your second domain to the DNS servers of your webhost (allow two days before your secondary domain starts working).
To check if everything went OK, go to a "server header checking tool" such as
http://www.seoconsultants.com/tools/headers.asp and type the name of your second domain in the search box. In return, you should get a message saying something like "Status Code HTTP/1.1 301 Moved Permanently". Make sure you see the "301" number, because it will indicate the search engines that your second domain doesn't host duplicate content but is simply pointing to an existing website. This should be enough to avoid any penalties and to transfer any Pagerank your second domain may have to your main domain.
Hope this helps,
Mario Sanchez
The Internet Digest
http://www.theinternetdigest.net