Having just read Horsemen of the Apocalypse, I noticed a variant of Death Knell that could kill several individuals at once. If it did, would the casters bonus' stack? I suspect that the answer is no, at least for the hit points (temporary hit points are not supposed to stack) and the strength enhancement bonus (same type bonus' are not supposed to stack either. The bonus to caster level is untyped, and therefore potentially stackable, but a few slain enemies could give you an insanely high CL.
So my original question, do they stack?