It's all broken down. If a weapon does 20 physical, 20 fire, 20 cold, 20 poison, 20 energy against a monster with 40/30/70/50/40 resists, then the total damage is split up first then applied against the specific resistance to give the damage inflicted.
Say your total damage output is 50 points, and your weapon is 50% poison and 50% fire. That means 25 points damage against the opponents poison resist, and 25 points damage against the fire resist. Depending on how much resistance they have is what final damage you apply to them.