The only real problem with this approach is that you almost certainly will hate trying to work normally using "Horizontal Span" since all windows maximise across both displays, and popups appear centered on the divide between the monitors.
that is not totally true. if you dig in a bit deeper you'll find options to make apps resize to the window they are in, and for popups to show centered, in the middle of the active screen, in the middle of the application screen or something like that. Worked for me, untill one of my monitors died (and I haven't bought a new one yet :/ )