Well i can answer to your Question as this is one of my recent research and testing results. Firstly let me see did i get you correctly or not. You are trying to have parent portal and child protals.
Well i can tell you its easy so the solutions has a steps and a few limitations also. Firstly i will tell you about the steps.
-- You will have to go into the hosting control center of Godaddy and then go to File manager and go to your DNN folder where you have installed.
-- Select all the files and folders COPY them to your ROOT. This can be done and only after this your multi portal senario will come into effect.
-- Then go to the host account of DNN site and make secong protal www.2nddomain.com and on godaddy you need to go into the DOMAIN CONTROL and map that domain name to the root where your DNN files reside. ( Please dont point it to the dnn folder point it to the root) and point the primary domain also to the ROOT.
-- This Process will take about 20-30 min and then try typing your www.primarydomain.com and www.2nddomain.com. It should take you to the new site portal now. If in case it is still not coming then go into the hostsetting then performance setting and clear the cache. After that it should work.
Restriction will be you cannot host any site like HTML pages from the root then.
Let me know if you get this right. If you need me email me mujahed_khan@yahoo.com
Thank you