As far as I know, you'd have to duplicate your rendering objects. Put everything into a sub-patch and make a copy rendering to a different named destination. There may be a better way though, I am not sure.
Of course, a simpler way to solve this particular dilemma would perhaps be to just mirror the two displays in system settings, as long as you didn't need to be able to see any other controls or information on the laptop screen.
for opengl you can use a shared_context.
one way is to capture your scene to a gl.texture (using gl.sketch can help with this), then create two gl.videoplanes, one on the main context and one on the shared context. pass your capture texture to both videoplanes.