Looking at these results there appears to be a simple multiplier effect from the second ingredient. The base damage of the poison is 36 when the multiplier is 1. Deathbell appears to have a multiplier of 1.5, river betty a multiplier of 2.5, and jarrin root a multiplier of 100 (the slight differences in the actual damage may come from a rounding effect). Perhaps certain ingriendient have a multiplier applied to one of their effects. When creating a potion using two ingredients that share an effect, the system will apply the highest multiplier found on one of the ingedients. I good test would be to combine river betty and jarrin root (multipliers of 2.5 and 100 respectively) - this should result in either a poison that is 100x stronger than standard (if only the highest multiplier is applied) or a poison that is 250x stronger (if both multipliers are used)
Interesting indeed. I will try to add these multiplier factors to the http://skyrim.melian.cc/?cmd=cmdSkyrimIngredientList and the http://skyrim.melian.cc/?cmd=cmdSkyrimAlchemyWizard of my site, once the construction set is out.
Nice find OP and all others who delved into this!