When both the attacjer and defender can refill dice what is the order for who re-rolls first? I'm assuming the attacker has to decide and re-roll first then the defender but just want to be sure.

Also can HK Assassin droids trigger a re-roll of the defenders die first, see the outcome of that result then decide if they want to re-roll one of their sttavk die? I'm also assuming yes but I'm playing in my first tournament at the end of the month and want to have a better understanding of timing effects.

Thanks.

Attacker first, then defender.

"Timing", RRG, Page 2:

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.

"Conflicts", RRG, Page 9:

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.

Attacker *effects* first. Meaning that if both re-rolls are coming from the Attacker (as is the case with all HK units), the attacker may choose the order in which the effects are resolved.