Dried cactus and stonecutter

How do these effects stack? Both are bonus dmg against fal. So would it be additive or multiplicative. Oh and I meant the rotten toadstool...

The bonus damage against fal from Rotten Toadstool and Stonecutter's Temple is additive. Same for Shadow.

Ok thank you for the clarification. Fuzzy brought up another point how does elemental bonus factor into this. Is the bonus from vex to fal also added or is it a flat multiplier?

Oh that is a flat multiplier indeed :-)