So the effects of snap trap and infestation dont stack?

Ok, so not sure why any sane person would even try this but I'm trying to make stunfisk work and was messing about with toxipex and g.stunfisk.

The idea was to toxic/yawn stall and whittle them down with infestation and snap trap. Spoilers, it sucks ass.

Anyway, I was surprised to learn that they don't stack. I was expecting the residual after turn damage to stack with each other but nope.