Answer
On Apps10, this is much simpler than horizontal clustering — you don't need to generate a cluster ZIP and copy it to a separate slave node. Instead, from the IFS Middleware Server Admin Console, simply add a new (Managed) Server directly to the existing master node. Leave the listen address field empty, and use the same keystore password you use to log into the Admin Console itself — ports are automatically assigned based on the instance's existing port prefix, and memory limits are automatically inherited from the first Managed Server. Within a few minutes the new server joins as a vertical cluster member. The main thing to verify beforehand is that the host machine actually has enough spare CPU and memory to support running an additional Managed Server alongside the existing one(s).