Asgorath
Explorer
So spells with an 'instantaneous' duration but with multiple beams/attacks, it cannot be that you could resolve the first beam, have your character wait to see if this kills the target, and then use the information to either attack the same creature with the second beam if it is still alive, or switch targets to attack a different creature with the second beam if the first is dead.
This is because the observation of the results of the first beam must occur after the beam's instantaneous existence, and by that point in time the whole spell and ALL its beams has come and gone.
From these forums it appears that many 5e players play it as if the spell had a duration of '1 round' during which you have several beams to use. This astonishes me.
This is addressed in the Sage Advice Compendium:
When casting a spell that affects multiple targets, such as scorching ray or eldritch blast, do I fire one ray or beam, determine the result, and fire again? Or do I have to choose all the targets before making any attack rolls?
Even though the duration of each of these spells is instantaneous, you choose the targets and resolve the attacks consecutively, not all at once. If you want, you can declare all your targets before making any attacks, but you would still roll separately for each attack (and damage, if appropriate).
Spells like Magic Missile that explicitly say they strike their target simultaneously mean you're supposed to do a single damage roll and thus apply of effects like the Wizard's Empowered Evocation and so on.