If it's a high end flashlight the brightness will be the same either way, you'll just get double the time. It will have a high efficiency constant current regulator driving the LED and what matters is watt hours.

If it's a cheap flashlight you might get almost anything. It's not uncommon for a cheap flashlight to be brighter with two cells because it doesn't have a boost converter built in so the higher voltage affects the brightness.

But OTOH the cheaper flashlight might result in just more heat, if it has a really junky regulator it might take the excess voltage and just bleed it off as heat. It would have to be real crap to do that, even $10 flashlights are better than that.