Why Create Two Exposures?
You’ll likely need to create multiple exposures when there is no moonlight, or if there is light pollution. Your earth and foreground will be underexposed/noisy, even if you brightly expose for the night sky. When shooting two exposures, shoot one exposure for the sky and another for the earth. For the sky, use the 500 rule, and for the earth, choose your settings based on noise and depth-of-field.
Blending the exposures in post will be covered in a later chapter.