There is a useful rule in the PHB concerning multiple creatures being damaged simultaneously (PHB, p. 196):

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.

That's a handy rule that stops us from needing to roll damage dozens of times for a single fireball. But it got me thinking about the Sorcerer's twinned-spell metamagic (PHB, p. 102).

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).

It has been clarified elsewhere that you aren't casting a second spell, but rather including an additional target in the effect of the original spell. And while it's been established that the attacks from a twinned spell with attack rolls don't happen simultaneously (specifically in the case of Booming Blade), it seems less clear to me that this would be the case for spells with no attack roll.

So if a spell like Blight were twinned, would you only roll once for the damage and use that total for both targets (modified by their respective saving throws), or would you roll damage twice (once for each target) because the original version of the spell only targeted one creature?