I have Zephyrus Duo and cannot figure out how to make an application go fullscreen using both internal monitors. Screen Xpert will only let you do it if both monitors are the same resolution and (of course) they are not. This is killing me.
I think you mean configuring a 'virtual' screen that spans across multiple monitors? You can do that with nVidia Surround, open the control panel and go to the "Configure Surround, PhysX" which should be the 3 option under the '3D Settings' node.
I'm not using the second display of the Duo as it's currently hooked up with 3 external monitor, but same thing would apply, in this example I've created a screen #3 which is made spanning my 2 1080p monitors resulting in a 3840x1080 pixels virtual screen. Apps would maximixe across both physical monitors as it would be one.
I don't think the monitors need to have the same physical resolution, you could set a scale factor on the higher res one to match the -horizontal in your case- resolution of the main laptop screen, and from there configure the spanning with nVidia surround.
That's your use case? maybe I didn't understand it right. What app would you benefit using in this mode?