So the tap ability triggers the second ability twice? That seems quite strong even on its own

I had hoped "or" would make it so the trigger only occurs once. Is there a way to word it so that the trigger only occurs once?

Not without changing the interaction with other cards e. g. the easiest way would be to remove "Commit heresy." from the activated ability.

Maybe you could do something like:

• ": Sacrifice a creature. This is an act of heresy.
  Whenever you sacrifice a creature and/or commit heresy, do the thing."

Actually there was once that one Azorius ability concept I've been working on with some people. It worked a bit like a static ability e. g. "Discarding a card is against the law." and a triggered ability "Whenever an opponent breaks the law, tap up to one permanent that player controls." etc. It was problematic because white-blue actually are not that good at punishing and laws were about your opponent doing things. I think, the same technique though might work wonders for heresy.

Three abilities:

• "Sacrificing a creature is heresy.
  : Sacrifice a creature.
  Whenever you commit heresy, you may distribute two +1/+1 counters among any number of creatures."