I wouldn't call it an active/active cluster. It is really a multi-instance cluster in the Windows/SQL Server world.
To create a multi-instance cluster, you need to create separate resources for the new instance. That is, you will need an IP address/name, separate disk drives and a new group in the cluster.
Once you have all of those, you just install SQL Server and select those new resources for the new instance. After the install is completed - move the group to the node you want to be 'active' and you now have your active/active cluster.