Is it possible to inflict multiple doses of poison simultaneously with a single attack? I know you can't normally put extra doses on your sword, but what if you had an Imp familiar, with a naturally poisonous sting on its tail. Could you apply a mundane dose of injury poison to his tail? Could you then also cast poison (a touch spell) and have your familiar deliver it via a tail sting, thereby potentially hitting your foe with three different sources of poison simultaneously?
As long as they're different poisons, I don't see why not.
Sure.
As much as I'd like to say no, I can't think of rule that would say no.
However, wouldn't the imp poison himself unless it was injury poison (when applying the poison to himself)?
Certain rogue talents also give you the possibility to apply more poisons to a weapon.
I'm aware of the rogue talents and what not, that's how I know you can't (traditionally) do it with a weapon and multiple simultaneous doses of poison on the blade.