One of the dangers of simul-climbing is expressed here in a particularly scary way:
In simul-climbing it is much, much worse if the second climber falls than if the first climber falls. If the first climber falls and the gear is solid, it can be very similar to a regular belayed fall, though much scarier and potentially much more serious. But if the second climber falls and the leader is any distance above his last piece, it will likely kill the lead climber. This is because he will be pulled the last piece of protection and then abruptly stop - not getting the advantage of the dynamic rope which stretches to cushion the impact of a fall.
The placement of a Ropeman device here and there can theoretically prevent that worst case scenario.
Best wishes to the victim for a full recovery.