If you are still having this problem, I know someone who has said this to me about my own sites and can likely help you:
If you decide you want the sites to be identical they can be set up with forwarding (easiest and cleanest) or if they are on the same host you can use the same database with the same information in wp-config, both sites must have the same theme and plugins installed. Otherwise they can certainly be set up independently...
He is the author of this WordPress plugin I use:
http://www.whereyoursolutionis.com/simpler-user-profile/