This does not sound good. I would take a good look over the coop and make sure there isn't any problems in the coop. Is there feeder off the ground cause rodents will come in at night and eat from it and that could spread disease to them too. Look for mouse poo, also like others have asked do both pins get food from the same bag? Make sure you are not using anything hazardous around the coop cause they will eat it and that could have done it. Do you clean out there coop regularly and does it have good ventilation. If not it could be ammonia build up in to coop which is bad for them. If i can think of anything else ill ask.