The SUBCISPAGE2 control is not officially supported for NATPAGE layouts.
The reason is: You ususally would specify a relativ url. But when doing so you end up in the same CIS session/subsession which is already bound to the Natural application containing the SUBCISPAGE2 control. This won’t work.
If you specify an absolute URL (http://localhost:8080/cisnatural/servlets/StartCISPage?PAGEURL=…
you will get a new session/subsession. But you usually don’t know the absolute URL on the Natural side.
So, sorry, not officially supported for NATPAGEs yet.