Curse reduces the str/int/dex of mobs, which as 5% says, lowers the barding difficulty.
Discord lowers the resists and skills (it used to lower stats as well, but was changed), which also lowers the barding difficulty.
Both can be in effect at the same time, but the only attribute affected by both is the barding difficulty (barding difficulty is based on the total attributes of the mob), and that definitely stacks.
This is for PvM. Discord doesn't work on players and curse works differently on players.