Problem installing SQL Failover cluster Let me first tell you my current environment in which there is one windows cluster on which there is sql cluster with two nodes, i.e, A and B in it. Of which, one A is active and B is passive. 
So, what I want is to use my passive node, i.e, B for some other application's database and make it work like active node for this application and A which will act as passive in this case.
Using this I can utilize my one server which is free at a time.

The point is that can we do that?
Yes you can do that with Failover Clustering. But you will need to license this second instance. 

If one node fails and can't be brought up quickly, you need to consider whether or not the hardware can support both instances for a period of time. In a 2-instance scenario, it is generally advised to have 3 nodes: active/active/passive.