NCS process the Northbound HTTP requests serially, and it will never get into the conflict of objects at the same time.
What is the scenario where NCS responds 409 error code for the NB Restconf API?
What happens if there are two clients on the Northbound, where one integrated with RestConf API, and other integrated with Netconf API? Will there be a possibility of a conflict of objects in two different types of requests?
Multiple northbound interfaces to NCS would not lead to an inconsistent database. When requests reach NCS from multiple interfaces, they will be serialized at some point towards the database. Are you seeing some issues with multiple North interfaces?