If you are going to need to have multiple Author instances setup for your Adobe Experience Manager infrastructure (AEM 6 and higher), then you are going to need to cluster them with MongoDB. This can be a difficult process. Javier Reyes Alonso and Marco Pasini from CodeBay Innovations have put together some helpful instructions about how to cluster your Author instances with MongoDB. This isn’t a complete step by step process but some general guidelines. Their article also contains some helpful links that can give some deeper intel into certain areas of this process. Go check it out.