What about a new ip address and service accounts, as currently running two nodes that are active
Of course each new instance needs to have it's own cluster resource group, including IP-adress and shared disk. If you want to use another service account or the same as for the existing instance depends on your internal policy. I've seen companies there they use one account for all instances andothers have a different account for each instance.
What I meant to say in my first post is basically you have to follow the same steps as when you installed the first instance except for the fact that you have a diferent name and cluster group.