The Screen Configuration app allows you to overlay the two displays, therefore giving you a clone (that tends to be the default).
What isn't exposed by that application is scaling the screens should they be different resolutions. The simplest approach to solving that is to change the resolution of the display. Alternatively xrandr does support a --scale option, however that resize is fairly inefficient and hit performance quite badly when I just tried it (xrandr --output HDMI-2 --same-as HDMI-1 --scale 2.0x2.0)
Software Engineer at Raspberry Pi Trading. Views expressed are still personal views.
I'm not interested in doing contracts for bespoke functionality - please don't ask.