The two exposures occur simultaneously, one by the ambient light, the other by the flash illumination. You can vary each of the exposures for a wide variety of different effects: You can make them of equal brightness, a little unequal, or widely unequal; you can give them different color balances; you can even vary shutter effects, with one exposure motion-blurred and the other tack-sharp. This is what makes flash photography so much fun—and so supremely useful.