Hello, when i reroll up to 2 attack dice from range 5 on, can i First ROLL 1 DIE and then if i want, ROLL the 2nd DIE - IN SEQUENCE - or i am obliged to reroll both Dice simultaneously?

Deleted.

Since the rerolls come from ONE ability, you have to choose how many will you rerroll. So yes, if you want to reroll both you need to do simultaneously.

You don't have to do it simultaneously only when the reroll come from different abilities.

The answer is in the FAQ, btw.

You perform the Elite Sniper ability only once, you choose how many to reroll, then reroll them.

FAQ3.0 said:

Q: When using "Elite Sniper" with an elite Alliance Ranger, do
I have to reroll both dice at the same time, or can I reroll the
second after seeing the result of the first?

A: If you choose to reroll two dice, both must be rerolled at the
same time.

Each different reroll ability is performed one at a time, so if you have rerolls from multiple abilities, you know the result of one before deciding on the next.