MongoDB temporary node / mirror
You can replicate data from one server to others using Replication feature in MongoDB.

The replica set ensures high availability and redundancy of you mongodb data in case if node failure. In your case, you can create a replica set of 3 nodes (PRIMARY, SECONDARY, ARBITER). When one of the server goes down, Secondary would be promoted as PRIMARY which ensures high availability of MongoDB data.

It is always recommended to have at least 3 members in replica set.

You can find detailed document here