I recall Frismund and Ahtiehli fighting, and they had no problem using it on one another. But that was lich vs. mummy, so was that the reason? You can't disrupt the type of negative energies you use yourself?

Not that I imagine this is a frequent scenario where two undeads fight, but is there something that needs to be present in order to allow it to go through?

Cause I can't imagine what necessary skill Tavlin, or Cotzibue for that matter, would not have had mastered.