Let me wrap this a different way and tell me what you think. Let's remove the illustrative trappings for a moment.
Essentially we have 2 monsters, each of whom has a special attack that looks something like this:
"Force all heroes to roll X Dice against Y Target Number or they are unable to spend hope to add Z Static Bonus to their skill rolls."
If the form of the above attack's delivery was:
-an area Psychic attack called "Morale Block ", would you omit one of the monster's attacks because the other one had it?
-a Venomous area of of effect spit attack called "Soporific Musk", would you omit one of the monster's attacks because the other had it?
Again, if you (generic "you") want to avoid making (# heroes) x 2 fear tests at the beginning of every round, sure, I get that. I'm just looking to provide some information about what's being omitted. And I would certainly submit that +1 to the TN to represent the combined coalesced "fear factor" is the proper way to go because otherwise you're just entirely stripping out one monster's special attack (which doesn't seem fair to that poor little monster who worked so hard to be terrifying).

EDIT: Also, Rich, in light of recent other threads that have cropped up recently, I want you to know it's my intent here to be delivering my sides of this discussion as calmly, honestly and respectfully as possible. If at any point that gets lost, you have my sincere and immediate apologies.