Ready action between multiple hits

  • Scenario 1 : An ally is unseen and has multi-attack. He has advantage on the first attack of his Attack action and I want to use the Help action to give him advantage on the second attack of his Attack action.
  • Scenario 2 : I am a lvl 5+ Warlock. I have a familiar, I am unseen and want to get advantage on both beams, so I want my familiar to use the Help action for the second beam.

Is it possible to Ready the Help action with trigger “after the first attack/hit of my ally on this particular ennemy” ?