Can I install another NKLS to do a redundant server in case of the main server is down?

Yes, you can. But please note that each NKLS must have the Network Key installed (e.g. for 10 concurrent users). However, a redundant server does not communicate with the main NKLS. They are actually the same. Hence, it is possible that the client might violate by setting up more than 10 concurrent users via 2 license servers.
Here is the practical solution. You generate a network license for the main NKLS with 10 concurrent users. You would generate another network license for a redundant server with 10 concurrent users, but limited max executions allowed, e.g. 100. This way, the redundant can be used for a limited time only until the issue on the main NKLS is resolved.