I'm not sure if this interpretation of RAW is correct or if there is any RAI that would decide if twinning a warcast spell is valid. Is there anything that makes it explicit that a single target spell cast through the War Caster feat can or cannot be twinned?
War Caster
When a hostile creature's movement provokes an opportunity attack from you, you can use your reaction to cast a spell at the creature, instead of making an opportunity attack. The spell must have a casting time of only 1 action and must target only that creature.
From what I can tell, War Caster takes place during the triggering of the reaction. You replace a opportunity attack with the casting of a spell, or essentially the Cast Spell action. Once you select a spell that takes 1 action and is targeting only the creature that has provoked the Reaction, the requirements of War Caster are satisfied.
Metamagic: Twinned Spell
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).
Based on the phrasing of this metamagic feature, you select to twin a spell after you have selected targets. That would be after War Caster's conditions have already been met.
Now, Twinned Spell does not work on things like Fire Ball, because it can target more than one creature, even if it currently doesn't. I'm not sure if this can be applied to War Caster in the same way though, because what is causing that to be the case is Twinned Spell. And if you can't use something like Fire Bolt because it could be twinned and that makes it violate the condition "must target only that creature" that would imply that you can't twin Fire Bolt ever because "that targets only one creature" would also apply. So if you can't twin a warcast spell, you shouldn't be able to twin any spells, from what I can tell. That leads me to conclude that you must be able to twin a warcast spell. Twinned Spell does not apply retroactively to itself, so it would make sense it doesn't apply retroactively to War Caster.
What information exists that makes this clear and less reliant on just trying to guess the order of operations for when War Caster's condition stops applying?
Best Answer
Probably not
You select the Twinned Spell target at the same time you select your primary target for the spell from War Caster. It says "When" you cast the spell. In other words, there are not two rounds of target selection, only one.
You can cast a spell that does not require an attack roll, but I think looking at the rules for making an attack helps reinforce this interpretation:
Set aside the question of whether or not twinning is allowed for a second. The reaction is resolved by first declaring targets (at this point, the caster would pay the sorcery point(s) to add a second target if it were allowed), determine what sort of bonuses or penalties each roll would make, and then roll the dice to make the spell attack.
Obviously, the spell might not involve an attack roll, in which case the latter steps are skipped (or they apply to the target if it needs to make a save).
But we see only one round of target selection because it's one spell. Twinning just gives the caster the option of expanding that selection to more than one target, but War Caster precludes selecting more than one target for the spellcasting. All this is established before the spell's effect is resolved and the casting is finished.
So I think the rules indicate that one can't twin a spell cast using the War Caster feat.