Understanding of SQL Server active/active cluster
You've essentially got it right.  Active/Active is really just a multi-instance SQL cluster.  When you install the Failover Cluster Instance (FCI) I1 on N1 of the Windows Server Failover Cluster (WSFC) you then have to run Add Node on N2 of the WSFC for I1, and vice versa for Instance I2. The SQL Server and Agent services are installed on both nodes, but will only be actively running on the node that actively has control of the SQL Instance.