What happens if during an attack, the attacking player as well as the defending player may reroll a dice?

Timing issues are resolved in the following order (unless specified otherwise).

1. Mission Rules

2. Player with Initiative

3. Player without initiative.

In your example, it doesn't matter who is attacking and who is defending. The person with initiative rerolls first (timing step 3 of an attack).

Timing issues are resolved in the following order (unless specified otherwise).

1. Mission Rules

2. Player with Initiative

3. Player without initiative.

In your example, it doesn't matter who is attacking and who is defending. The person with initiative rerolls first (timing step 3 of an attack).

Who has initiative in a campaign mission? Is it the figure that is currently activating?

They probably should have had a rules entry for "Timing", but instead you have to look under "Conflicts" on page 9:

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).

It's only when there's a timing conflict outside of an attack that the priority goes by Initiative (in a skirmish) or to the Imperial (in a campaign).

edit: To the OP's specific situation, remember also that each physical die may only be rerolled once, so if the attacker does so then the defender may not reroll that specific die any more, but may reroll some other die if they have that ability.

I created this timing structure for Imperial Assault Skirmish like the other FFG games. Hope this helps!

That's what I was expecting. just checking. thanks