You are correct - any data submitted on one publish instance is not automatically replicated to the other publish instance/author without some additional configuration and/or code. There are numerous ways to achieve this functionality but implementation really depends on what it is you're trying to do. You can certainly configure AEM to automatically replicate data when a node is created/modified without adding any additional code, but you need to ask if that's the best implementation for what you're trying to do.
As far as MongoDB, if you're talking about for User Generated Content then you're likely referring to MSRP. It's important to note that use for this requires a license for Adobe Social Communities and any custom functionality will need to extend the appropriate API's to store and read data. You're also correct that there wouldn't be configuration required to keep the publish instances in sync, but, there is some up front initial configuration for the datastore. It should also be noted that you'd need to setup a SOLR instance for MSRP too.
Could you provide some more information around your use case so that a better answer can be provided?
