I have a chick with a sore eye too - I think she has just hurt it somehow (poked in the eye by another chick when playing perhaps). We've been rinsing it out with a saline solution (1/2 tsp uniodised salt to 1 cup water boiled for 15 minutes with the lid on the pot then cooled and stored in a clean lidded container). I've been using an eye dropper. She's the same as yours, eats, drinks, is active, but I know she's feeling sorry for herself and is a bit more sleepy than usual which is understandable if she's in pain and is healing. If she wasn't eating I'd start to worry. The other chick is absolutely fine. Our girl does close both eyes (especially when we get her out to rinse out her eye!) but seems to be improving - I see her opening her bad eye more and it still looks a bit gunky but we are getting there. It would probably stress your chick out more being separated from her friends. Hope that helps.