I run dual displays, one 3440x1440 100Hz and a 2560x1440 60Hz never have had this issue.
Running in Fullscreen mode defeats the purpose of having two monitors, as you still have to tab out to go to the second monitor and in most if not all cases it still minimized the game. You should be running Windowed (Fullscreen) with any game that runs it properly. (some games alter the resolution and refresh rate so that's not possible, in THOSE cases you have to run Fullscreen mode or Windowed mode) WoW runs Windowed (Fullscreen) properly so that's not an issue. And I'm assuming since Destiny 2 is an Activision game it should be similar enough that it should also run properly in W(F) mode.
I would reset the Nvidia Control panel settings to default (I run on default and have never had this issue), to make sure there are no power saving settings applied or whatnot, and possibly use DDU to clean your drivers and reinstall them.
It wouldn't hurt to check your power management settings as well and make sure that's not set to power saving mode or something. For my purposes I find "Balanced" works just fine and i've even had issues when running Performance mode, so you might have to fiddle with it.
Check to make sure that in Windows and NCP both monitors are set to run correctly (extended, mirrored, etc.) and that each has it's correct resolution and refresh rate set as well.
Lastly, might be a long shot, but make sure in WoW or Destiny that you have the background FPS setting turned off. There might be something like that for NCP as well, especially if you are running a graphic as your desktop background and software to control it.