By default, we strongly suggest that you activate ONE membership platform at a single WordPress installation. Activating two or more membership plugin will likely cause issues to your WP roles at your database.
You can use OptimizeMember (OPM), the membership plugin that comes with every package of OP, to protect your contents. To learn more on what OPM can offer, kindly refer to this link: http://help.optimizepress.com/article/173-optimizemember-feature-list
Or, you can use a 3rd party membership plugin such as DAP (Digital Access Pass) or Wishlist to protect the contents in your OptimizePress site. For detailed guide on this, please refer below: