Certain attacks are described as inflicting ‘dual damage’. This means that the mud will check both damage types each time you hit, and choose the one which causes the most damage, taking into account all factors (including bypass, worn armour, powers, talents, spells, and everything else).
For example: Bat Form inflicts dual cut/poison damage, meaning that each time you hit the damage type will be either cut or poison, whichever is better. If your opponent is wearing a steel breastplate but no head armour, you might end up inflicting poison damage to his body, but cut damage to his head.