Basically I've got two domains that I want to make display the same site.

So, if firstdomain.com/test.htm existed, you could type in seconddomain.com/test.htm. This needs to be a true mirror of the firstdomain.com site, and not a frame redirect.

Are these on the same server?

If so, why not just use a server alias? If you don't like getting redirected you can edit the httpd.conf file to remove the Rewrite rule.

Yes, they are on the same server, what line am I looking for in httpd.conf? And will the contents of httpd.conf be overwritten due to plesk at all?