Now that we're a little ways into our campaign, more figures are getting access to reroll abilities and I've started to see this kind of situation more often:

• One person involved in attack likes their roll and declines to use an available reroll.
• The other person uses a reroll.
• The first person, seeing the changed outcome, now decides to use their reroll as well (on a different die).

But I'm not sure this is technically allowed. According to the steps of an attack, the "timing instance" for all rerolls is concurrent:

RRG, page 5, "Attacks - Steps of an Attack":

Rerolls: If players have any effects that reroll dice, they are resolved now.

- Each die may be rerolled only once per attack regardless of who is rerolling the die.

Since both the attacker and defender have simultaneous options to reroll, that constitutes "abilities ... triggered at the same time".

RRG, page 9, "Conflicts"

Numerous abilities, mission rules, and other game effects can be triggered at the same time.

• While attacking, mission rules are resolved first, followed by effects from the attacker (including figures friendly to him), then effects from the defender (including figures friendly to him).

• Effects from multiple figures under the control of a single player are resolved in the order of that player’s choice.

• The decision for a player to resolve an optional effect coincides with the order of resolution. After a player has declined to trigger one or more abilities, he does not have the opportunity to trigger that ability until the next triggering instance.

Conflicts in a Campaign

During a campaign, mission rules are resolved first, followed by effects from the Imperial player, then effects from the Rebel players.

So it seems to me that in any attack, there is one player who must decide first whether to use a reroll, and if they decline, they cannot change that decision after the other player has decided to use a reroll.

The burden of initiative is a little ambiguous, though. Since this is "while attacking", the first bullet says it is the attacker who must first decide to reroll (or not to), no matter if it's an Imperial or a Rebel who is attacking. But "during a campaign", the Imperial always has to decide first, no matter if they're attacking or defending.

What's everyone's thoughts on this?

Effects resolve in a certain order.

Attacker rerolls first if he wants to. If he does not, his timing window passes and is unable to reroll again.

"Timing" page 2 RRG:

During an attack, in both a campaign and a skirmish, resolve mission rules first, followed by effects triggered by the attacker, then effects triggered by the defender

Ah, I missed that clarification of the priority of the conflict resolution rules. Pity they didn't restate that in the actual "Conflicts" section, I'd forgotten there even was a "Timing" section at the very beginning (I looked for it first under T but of course it's not there).