I would start them both on Corid. For the little one that is failing, I'd jump start her with a direct dose of undiluted Corid at .10ml per pound of weight, so for a tiny chick like this, I'd give her something like 3 undiluted drops or so. Do this twice a day for 3 days.
Liquid Corid dose is 2tsp or Powdered Corid Dose is 1 1/2tsp per gallon of water given for 5-7 days as the only source of drinking water.
Do not add any extra vitamins/electrolytes that contain B1(Thiamine) to food or water during the course of treatment.