My thoughts would on this would be:

First of all,we weren't there when it happened and we don't know all the details,so we can't make any judgement on what was the cause of the accident and how it could've been avoided.

some possible scenarios are:

If the anchor failed itself,then maybe backing it up if it looked old or slightly untrustworthy would've saved their lives. a piton,a nut or even a cam is worth way less than a life or two,but alot of climbers dont seem to think this way.

If one of them did not rig their rappel device properly,then knots in the rope may have saved one life if it got jammed in the anchor,but it is hard to say because the person still on the rope would take a decent fall,one long enough where death could still occur on certain terrain.

Checking their rappel before they unclipped from the anchor would've solved this.i almost had it happen to me at wasootch a couple years ago,BUT i remembered to bounce on the device before i unclipped from the anchor and then realized that in my rush to get down,one strand of rope was not clipped in. i wouldn't have died,but i would've got injured nicely.

If it was just them simul rapping and one of them was not used to doing so and lost control,then having knots in the rope might have saved them both from death,but not injury.

we dont know what happened for sure,but those are three of the possibilities and what would've resolved them. i am not that experienced with climbing,but i know enough to get by.

everyone makes mistakes when they are in a rush or tired and trying to get down a mountain.
You just have to remember that it is more important to take a moment and not rush down the mountain,unless you have equal chances of dying from just being up there any longer and that typically doesn't ever happen unless you are on a long harder alpine route.

Check your anchors,check your belay and harness,it only takes a minute.