If I use Twinned Spell to target two creatures with an attack, do the attacks occur simultaneously?

Question

Suppose you are a Sorcerer multiclassed with a Paladin, and you have Searing Smite active when you run up to two adjacent creatures, and spend a single sorcery point to use the Twinned Spell metamagic on the cantrip Booming Blade. You make two attack rolls, one against each creature, and both successfully hit. Which of the following statements is true?

• The first creature targeted is affected by the Searing Smite spell
• The second creature that was targeted by the Twinned spell is affected by the Searing Smite spell
• Both creatures are hit simultaneously, and you are given the choice of which creature to affect with Searing Smite (but you may only choose one!)

The practical use of this question is that, at the time you make the attack rolls, you have "locked in" your targeting order, but if the second attack roll results in a critical hit, you might be incentivized to prefer applying the smite damage to that target instead, gaining the additional crit damage on the smite. So resolving whether these attacks occur simultaneously or not affects what decisions you could make in this scenario.

Answer 1

They do not happen simultaneously

The optional rule on timing is in Xanathar's Guide to Everything (p. 77):

In rare cases, effects can happen at the same time, especially at the start or end of a creature's turn. If two or more things happen at the same time on a character or monster's turn, the person at the game table - whether player or DM - who controls that creature decides the order in which those things happen.

It's your turn, so you get to choose which of the twinned spells happens first and make that attack roll. If it hits, then Searing Smite will be triggered because it fires "the next time you hit a creature". Hit or miss, you then move on to the next attack.