I know I have experimented with this before, but I don,t know how it exactly works. Somehow it stacks with only 1 buff if you select one or more stacking options.
Also if I'm not mistaken, each poison stack has its own timer/duration, but I'm don't remember it was only for damage it did work that way. Its hardcoded in a way so I only know that I tested this years ago but I don't remember the stacking effects. So I'm not completely sure either...