Firstly, You need to understand the major difference between the Name-Node and The Secondary Name-Node.
Name-Node is designed to save the information of the HDFS filesystem.
Secondary Name-Node is designed such that it acts as a buffer to the Name-Node. Unfortunately, it does not act as the backup for the Name-Node as its name depicts.
secondary Name-Node stores all the updates obtained to the Name-Node in the intermediate computations and these changes are applied on to the system when the system gets restarted.
I will provide you a link to this information where get more detailed approach to the Name-Node and Secondary Name-Node.