He probably wants them to both be on the same portal.

This would give you the ability to make the one portal look like different sites depending on what domain was used to get there, which in turn could give you a back-end that combined the two different looking sites into one for a single sign on system.

This actually would not be that hard to make happen if you had a special skin placeholder that looked at the domain being used and loaded the desired skin.

The core code allows you to assign the skin based on a value in a cookie which you can read about here:

http://forums.asp.net/1/1067496/ShowThread.aspx

Nik even has an example of switching the skin dynamically which you can read about here:

http://www.dotnetnuke.com/Community/ForumsDotNetNuke/tabid/795/forumid/109/threadid/30217/scope/posts/Default.aspx

If I get a little extra time today, I'll work up an example of switching the skin based on the domain name.