if you have a DNN portal www.mydomain.com and want to add a second portal www.otherportal.com, you need to create the second portal in DNN as parent portal with the appropriate address (you might need to add the domain as host header to your web settings in IIS as well).

If you want to add another portal at www.mydomain.com/myportal, you need to create a subportal myportal in DNN and a redirecting subfolder is created aotumatically.

If you have a portal localhost/dnn and want to add a second portal localhost/anotherDNN add the appropriate parent portal in DNN and add another virtual directory anotherDNN pointing to the same address in IIS.