A droid blew up. It killed two dudes. One was MHD and I had the command card medical miracle.

Could I use that on the other guy that got killed? Or is MHD dead right away and you can't use it.

All figure suffer the damage simultaneously, but as far as we know, their defeats are performed in the order of the ability's controller's choice (because it seems to be the best order).

If MHD-19 is defeated first, he's no longer on the map to play command cards on when the other figure's defeat is performed.

That conflicts with other advice I’ve gotten on this from the Zion’s Finest Slack channel:

“I would agree it's simultaneous.

An anecdotal but similar case:
I e-mailed Todd to ask whether Vader moving adjacent to Han and Jyn could set off two copies of Slippery Target ... that is, one copy proc'ing off of "Vader moves adjacent to Han" and another proc'ing off "Vader moves adjacent to Jyn."

The response included the observation that "both Han and Jyn’s opportunities are triggered by the same event."

Based on that, I'd say that the death of MHD and the death of Luke are "triggered by the same event," and so there is no timing window in which damage is being dealt to JKL, but not MHD. That is, there is no moment where Luke is defeated in which MHD is not also defeated.

Your game seems correct to me: MHD's card interrupts in that moment, and can target Luke to heal him, then you return to the timing instance of the damage being assigned, and MHD is defeated.”

Who is correct?

It seems your question was imprecise, Miracle Worker doesn't trigger from a figure being defeated, it triggers from suffering damage equal to health, and explicitly overrides the timing of "being defeated because suffered damage equal to health".

So, you play the card when the figures (and objects) simultaneously suffer the damage, and the (hard) question of simultaneous defeats and abilities triggering from those does not even arise.

The Slippery Target example isn't really that similar; You are playing multiple cards triggering on the same event. So timing conflict is resolved as normal, in this case you would choose which slippery target to resolve first.

For Miracle Worker, a1bert has already pointed out that the wording in this case would be clear. As both suffer the damage simultaneously, the trigger to play the card happens before MHD is removed from play.