CWSOP 010 Yellowjacket has the trait:

Hidden Agenda: When a Friendly character is KO'd heal Yellow Jacket 1 click

Let's say I duo target Wasp (friendly to Yellowjacket) and (an undamaged) Yellowjacket with a ranged attack. I hit both. I then split and deal damage (in this order since I'm the controlling player):

2 damage to Wasp. (Ko'ing her)
2 damage to Yellowjacket.

Would Yellowjacket be able to use Hidden Agenda to heal 1 click?

No. Yellow Jacket is still on his starting click when Wasp was KO'd.

Incorrect.

Damage is dealt simultaneously.

So Wasp is KO'd and Yellowjacket is on click 3. Yellowjacket then heals to click 2.

DAMAGE DEALT
[C]All damage from a single effect is dealt simultaneously, unless otherwise specified.


This is why Mastermind works the way it does: a character that is dealt 2 damage from an attack *AND* 2 damage via MM is dealt a total of 4 damage; not 2 and 2 (each potentially reduced separately).