Forum:Two monsters both with effect that negates each other's effect

I've always had this question in my head every time I play Yu-Gi-Oh. What happens if there's a monster on my field that negates other monster's effects, and if my opponent's monster also have the same effect?

For example: Neo Galaxy-Eyes Photon Dragon Vs. Number 69:Heraldry Crest

Would both monster's effects be negated or does it depend on which monster is summoned first?