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?
