I'm not sure how the nVidia GTX 960M and the Intel HD Graphics 530 work together in my Inspiron L7559. I think the intel is designated as a secondary and the nVidia as a primary. I can't seem to find an explanation anywhere. If I right click the desktop and click on Display Properties, the Intel control panel pops up, not the nVidia. There is however the option to open the nVidia control panel. Are the adapters switched depending on the demand of the application? Do they somehow work in conjunction with one another?
It's the other way around. The nVidia GPU is secondary; the Intel is primary. All video data passes through the Intel GPU on its way to the screen -- even if it's processed by the nVidia GPU.
The nVidia GPU is called into use when needed or when you select it on a per-application basis.
There is no way to make the nVidia GPU primary - the Intel one is hardwired to that role.