It’s really just framing one page in another, which I don’t really like to do. When they mouseover links, they’ll still see the real domain–plus the framing can make it hard for people to bookmark or link to certain pages.
You could add the other domain to your panel as a mirror, which would allow access under either domain name.
If the content is the same, just redirecting one to the other is a good idea… especially if you’re worried about any duplicate content penalties from Google.
I would make the site at the domain you like, then just redirect the other one to it.
Also note that some (most?) directories won’t accept submissions that frame another page or redirect to another page.
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