With a long slit orientated appropriatelyand a suitable image scale you might be able to get all you need in one shot (Well 2, one exposed for the bright side) You would need a flat field illumination source which is nice and even in the slit direction so the subtractions work accurately.
Funnily I was just reading Paul Abel’s beginners article on Earthshine and musing about measuring that spectroscopically, perhaps mounting the spectrograph behind a camera lens to get a sensible image scale to match the slit. Much easier than Ashen Light though !
Robin