How to add Nighthawk X6S AC4000 as second router to the C7000v2 model

> [...] I want to add the Nighthawk X6S AC4000 [...]

   Not a model number.  Look for "Model" on the product label.

> [...] as second router for my C7000v2 model, [...]

   Why?  Do you really want a "second router", or are you trying to
extend the wireless-network coverage of the C7000v2, or what?

   Visit http://netgear.com/support , put in your (actual) model number,
and look for Documentation.  Get the User Manual.  Read.  Assuming not
another router, look for "Use the Router as a WiFi Access Point".