You can do two WP installs, one for the front and one for the backend. Or look at
Adult Site CMS - Adult Content Management Software CMS Solution for Adult Sites which uses WP but I think they have to hack it a lot to get it to work both front and back-end, so not sure if it supports all themes and WP plugins, you would want to check with them on anything you want to have installed with WP to make sure it will work first!
Tim
