Oh, we've all been there. You're watching your little flock, feeling like a proud parent, and then, bam. One chick starts doing something weird. Then another. And suddenly you're googling "chicken gasping weak legs" at midnight, wondering if you missed a chapter in the chicken-keeping handbook. OP here has two chicks, a Mystic Onyx and a Midnight Majesty Maran, that are eating and drinking fine, but acting weak, gasping, and walking like they've got rocks in their boots. The poops were watery, now better after isolation. Temperature and ventilation are fine. So what gives?
Honestly, this is the kind of mystery that makes you want to pull your hair out. Or your feathers. The fact that these chicks are still eating and drinking is a good sign, it means they haven't given up. But the weakness and gasping, especially with no obvious leg injury or respiratory cause, is frustrating. We've seen this before in mixed batches: sometimes a chick just doesn't thrive, and you never find out why. But the combination of symptoms here, stretching wings down, holding them up, watery poop that improved after isolation, points toward something metabolic or nutritional. Maybe a vitamin deficiency that didn't show until now. Or a mild infection that's messing with their guts and energy. The fact that both chicks are from the same group but not all of them are sick suggests it's not the environment alone.
What we'd tell OP is this: you're doing the right things. Isolation, electrolytes, sugar water, yolk, that's a solid first-aid kit. But don't stop there. If they're still gasping after a day of that, consider adding a poultry-specific vitamin supplement with B-complex and selenium. Sometimes weak legs and gasping are linked to a deficiency in vitamin E or selenium, especially in fast-growing breeds. And check their crops, are they emptying overnight? A slow crop can cause that gasping look as they struggle to digest. Also, keep an eye on the rest of the flock. If any others start showing signs, you'll know it's contagious. If not, you can breathe a little easier.
Here's the hard truth: sometimes you do everything right and still lose one. That doesn't mean you failed. It means you're dealing with living creatures that don't always follow the rulebook. But don't give up on these two yet. Keep them warm, keep them hydrated, and watch for any change, good or bad. If they start perking up, you'll know you cracked it. If they don't, at least you'll have ruled out the obvious. And when you figure it out, come back and tell us. Because we've all been that person staring at a chick and whispering, "What is wrong with you?"