Ahh, yes I guess I should have explained that a bit better.
www.iowacomptuergurus.com is my regular domain, non-DNN based. portal.iowacomputergurus.com is a regisered sub domain, and it is it's own virtual directory....on my m6.net hosting account Ihave the following...
<MyFolder>\www.iowacomputergurus.com\www.iowacomputergurus.com\ - In here is my regular site
<MyFolder>\portal.iowacomputergurus.com\portal.iowacomputergurus.com\ - In here is my DNN site
So in your case, you would want to set up your DNN install under
myhostingrootdirectory\firstsite.com\firstsite.com
Then all other sites will point to that same location, and you will have NO additional file system entries for the other portals (Also, it will not count against your hosted domain count for M6.net :) )
You are correct with one install they share one database and they share the available modules and skins (if loaded from host).