Discarding in multiples is one of the few things in the game that happens simultaneously. Otherwise a card like
Siege Engine would be in trouble, because it could protect only one machine before discarding itself.
Not sure if there's a rule set somewhere, but entries like these provide precedent:
When an effect discards "all" minions (thereby discarding their weapons), they are discarded at the same time. None of those minions may have weapons played on them with
Plundered Armories.
The effect of this condition's special ability when the card has more than one token is simultaneous. Several conditions are discarded at the same time.
Siege Engine responds to these discards by preventing all of them.