Do multiple hits of Repelling Blast happen simultaneously or in succession?

A 5th level Warlock casts Repelling Blast, firing 2 beams at the same target. The first one hits, pushing the target back 10 feet. Does the second attack roll come before or after the target is pushed away?

Two scenarios (both came up in play):

  1. The opponent is a ghost who is 5 feet away from a wall. The first blast pushes the ghost through the wall. If the second attack is before the repulsion, both beams hit. If it is after, the wall would block the second beam.
  2. The opponent is an ordinary creature that is 5 feet away from the caster. The first roll was taken with disadvantage. If the second attack is before the repulsion, then that attack is also with disadvantage. If it is after, then the caster is no longer within 5 feet of an enemy, so the second attack is not at disadvantage.

This question is inspired by this other question.