General rule of thumb is that like ailments/DoTs/buffs overwrite if they have the same icon.
If Gormek attacks 3 enemies and defense is down is applied to them then Grimm attacks same three enemies defense isn’t further reduced by overwritten to apply the effects of Grimm’s special skill.
However if Grimm attacks 3 enemies and Nordri attacks same 3 enemies then you will see the elemental defense debuff as well as the standard defense debuff
@DaveCozy keeps a list of buffs and ailments in the thread below: