In the case of the DS at least, the big differentiator is that the second screen is touch, and is positioned in a more touch-friendly place.
If your one-and-only screen is touch, you have to lift your arms up away from your buttons to press anything. No matter how much of an impost it is to have to look down a little, I'd say it's still less than having to lift your hand away from the buttons into your eyeline.
If your one-and-only screen is touch, you have to lift your arms up away from your buttons to press anything. No matter how much of an impost it is to have to look down a little, I'd say it's still less than having to lift your hand away from the buttons into your eyeline.