Can you "park" one of the names, pointing it at the other?
For instance, I have a domain name which contains an "American English" word. I also have that same name with the "British English" form of the same word. When speakers of British English look for the British variant, they are automatically forwaded to the American English site. But their browsers show the British English name in the location bar.
So I have two sites (sort of), but no duplicate-content penalty. Is that the sort of thing you're looking for?
If the domain (the content) is under the one domain name, with the other domain name "parked" and pointing at the one domain, then no, there will be no duplicate-content penalty, because the domain is considered to be only the one set of content. There is no actual second site with the same content.
This is my understanding, and has been my experience. I welcome correction or clarification from those more savvy.