It depends. You can stack two normal exposures to end up with a neg that is one stop overexposed overall, but that can easily be printed. You can also cut a stop from each shot when shooting to end up with a neg that is overall correctly exposed. However, if you do this, each image will be one stop underexposed, with all the effects that come from this. For instance, you may lose some contrast and shadow texture and detail. Then again, sometimes areas of pitch black are what allow double exposures to really work well, as you can stack bright elements of the second exposure onto dark areas of the first and get less clutter. It depends on what you are doing and what you want.