Yes, you can do this. I did a similiar thing for two sites - one was a paysite, the other a free site. Both had their own look, but shared the same database for content. You will probably have to use the global override feature which allows you to override database settings for the second install, but its a part of PhotoPost and I'd be glad to help you with the syntax.
Please do not PM me for support or sales questions. Thank you for your understanding.