The sorcerer's Twinned Spell metamagic option states:
When you cast a spell that targets only one creature and doesn’t have a range of self, you can spend a number of sorcery points equal to the spell’s level to target a second creature in range with the same spell (1 sorcery point if the spell is a cantrip).
To be eligible, a spell must be incapable of targeting more than one creature at the spell’s current level.
Starting at 6th level, when you cast a spell that deals damage of the type associated with your draconic ancestry, you can add your Charisma modifier to one damage roll of that spell.
It's known that Twinned Spell does not create another spell, it just targets another creature with the same spell. For example, you can concentrate on two haste targets because of that. On the other hand, for AoE spells, Elemental Affinity increases the damage for all targets; even if it applies to one damage roll, that damage roll is applied to all targets.
That said, does Twinned Spell use the same damage roll against the two targets, thus allowing Elemental Affinity to apply to both targets? Or does it use different damage rolls, and thus Elemental Affinity applies only to one target?
The description of damage rolls in the rules says
If a spell or other effect deals damage to more than one target at the same time, roll the damage once for all of them. For example, when a Wizard casts Fireball or a Cleric casts Flame Strike, the spell’s damage is rolled once for all creatures caught in the blast.
So, the question can be rephrased as: does Twinned Spell make the spell deal damage to both targets at the same time or not?