I think that the other dani is racing to the top eating all the food before the other one gets a chance, if I'm correct try sprinkling food in 2 spots! One spot for the other danio and the other spot for the skinny danio!
It could also be a disease! Like jourdy288 said, it could be a skinnying disease! When this happens don't add any more fish! In honesty if it is a parasite he has the other one will catch it sooner or later! You need to go to your local fish store and tell them about your danio and they should get you medicine for the fish. That said, you may have to look around the shop for it because fish stores will sell you anything! They have really bad advice
~Karen
Sent from my iPhone using Tapatalk