I've been meaning to make a "screw your gens" build on plague, but I've been wondering if these even stack at all.
Yes, they both stack with each other to increase the penalty's against Survivors.
Keep in mind though, that Dying Light has been tentatively fixed with its known bug. Meaning, the perk is still being fixed and isn't 100% confirmed to be fixed yet.
The bug sometimes crashes any Killer that uses the perk within a game after a certain amount of time OR tokens have been gather with the perk. After such time or tokens, the perk causes the Killer to crash. Crashes usually occur when after hooking several Survivors, or right after a Survivor has been hooked mid-way through the trial.
Yes they do stack. :)