Very good question. I’d say (judging from the logic of how Pumpkin Bombs stack) that the Theriac’s radius would be increased to 4 tiles. While extremely strong, the absurdly small chance of this happening (and the relative difficulty of aquiring the Theriacs) makes it fair enough imo… :)))
I’m always impressed with your sharp eye and attention to detail with questions like this!! Many thanks indeed and do keep them coming if you stumble upon any more C: