Might you be referring to Normal and Nightmare, the way the original game implemented these modes?
If so, that's not possible on Steam; the way Global handled these modes was to make them drop-down, so new maps would generate in the chosen difficulty. Therefore, each playthrough only goes through the entire game once instead of twice - since modes are now settings and not playthrough modes.
What's more, I don't believe it'd be anywhere close to easy to re-mod these modes into Steam given this setup. What might be more possible would be to set quests to reset on their own after some time, but that would only make quests perpetually available instead of allow for playthroughs on different modes from scratch.
