ETA: Okay, I missed that in the main body of the Intrigue rules; I withdraw my objection. (And so far, Intrigue is the only set in which it matters.) However, I wrote all of the below, and I'm not going to delete it:
Actually I'm proposing that this would be a sensible way of interpreting the printed rules:
1. RHO plays an attack card.
2. LHO and I reveal any reaction cards that are in our hands and which we want to reveal, simultaneously, and each activate them simultaneously. If either of us revealed more than one, we choose in which order to execute them.
(But wait! What about the "go in turn order" rule? ...The rulebook actually says "If an ability of a card affects multiple players, and the order matters, resolve that ability for each affected player in turn order." The revealing of Reactions is an "ability" of the Reaction cards themselves, not of the Attack card, and so there is no ability of "a card" that affects multiple players here. Actually, an alternative to 2 would be the following:
2': LHO and I reveal any reaction cards we want to reveal, simultaneously, and RHO decides the order in which they are executed.
This should be clearly incorrect, but it's what's suggested by the wording of this rule: "If multiple cards resolve at the same time on your turn, you choose what order to resolve them." The Reactions are multiple cards resolving at the same time.)