The Sorcerer’s Empowered Spell metamagic option reads
Empowered Spell When you roll damage for a spell, you can spend 1 sorcery point to reroll a number of the damage dice up to your Charisma modifier (minimum of one). You must use the new rolls.
While the spell Chaos Bolt reads:
You hurl an undulating, warbling mass of chaotic energy at one creature in range. Make a ranged spell attack against the target. On a hit, the target takes 2d8 + 1d6 damage… If you roll the same number on both d8s, the chaotic energy leaps from the target to a different creature of your choice within 30 feet of it. Make a new attack roll against the new target, and make a new damage roll, which could cause the chaotic energy to leap again.
If Sorcerer chooses to Empower Chaos Bolt and has a Charisma modifier of at least +2 do they need to re-roll all of the dice that they wish to re-roll at once or can they re-roll one d8 then, if it does not match, re-roll the other?
As an example:
The sorcerer casts Chaos Bolt and rolls a 7 and an 8. They use Empowered spell and re-roll the 7 getting a 3. Can the sorcerer re-roll the 8?