How to add data from one Drupal site to other?
You need to migrate the content of site 1 keeping the config of site 2. My recommendation is to use migrate module coz I prefer it. But you'll have to write code to use migrate module. 

There are other modules that may help you like node export, deploy. You can also write custom script but I have had most success with migrate module. You might need to justify the use of migrate in comparison to the content to be migrated, because you'll need to write migration classes but for lets say 300 nodes I'd still use migrate.

I wrote an answer sometime ago which talked about migration of content, users, categories etc. See if it helps you to choose as well - Updating content after major upgrade from 5 to 6 (changes to production site need to be reflected in dev environment)