Julian,

if I understand what you're asking, this is easily accomplished. If you have already yourDNNsite.com/portal1 and yourDNNsite.com/portal2 created, then register the domain names you want to use and get them set up to aim at this instance of DNN. (set up the nameservers for these URLS at the registrar, and create virtual directories on  your webserver.)

In DNN itself, go to host | portals and select a portal. At the bottom of the screen, add an HTTP alias which will be www.yourNewURL.com for this portal.

Now (after DNS has propogated a bit) this URL will take users to the same content as www.yourDNNsite.com/portal1. In fact, you can still access it either way, since each remains an "alias" pointing to the same portal.

is this what you were after?