New gecko isnt looking so hot :(

So I recently just got 2 more leopard geckos from someone on craigslist and one is a healthy looking 6 yr old female and the other is supposedly 2 years old, but is way undersized. I've noticed they both had a little skin stuck on them from the last shed as well, but have shedding hides now. The sicker looking one also has something wrong with her eye lid, it sags really low on one side. On the brighter side, once i got them into their separate enclosures the 2yr old one immediately started eating and drinking as well as some of my others despite how weak she is. So this is my first time attempting to rehabilitate any reptile. But is there much more i can do other than try and get her some vitamin/calcium supplementation and keep a close watch on her?

Quarantine and proper husbandry are the most important things for them right now. Which from the looks of it, you got it all taken care of. So good for you

If she's eating and defecating now after being separated, I'd just keep at it for awhile and see if she's able to gain some weight. It'll be a good idea to track her weight so you know whether she's gaining or losing. Rapid weight loss can be signs of parasite infections, which a fecal exam should confirm that. But for now, give her some time alone to herself, and keep feeding her properly gut-loaded & dusted feeders. See if she can gain some weight and becomes alert and normal again.

As for the eye, can you take a pic?