Ok so it sounds like you are off to a decent start.
1) you NEED a better brooder. Homemade, purchased, or borrowed, what you are doing is not stable enough by a long shot and will cause major problems. I'm the mean time the sick one has very little chance of recovering if he isn't kept warm enough. You need a system that lets you monitor temp and keep it extremely consistent. This is an URGENT PRIORITY.
2) stop feeding the one with the slow crop. If you have an avian vet qualified to flush the crop or a local breeder (NOT the pet shop that thinks that crop looks empty; those people clearly are not qualified) who has experience doing it, have his crop flushed ASAP.
I don't suggest flushing it yourself if you have never done it or seen it done. It's better to have that demonstrated in person rather than trail and error.
If you can't have his crop flushed, start feeding fluids ONLY. Feed at the same temp as formula. I suggest coconut water or half water have PLAIN pedialyte or if you can't find that, Gatorade. Pedialyte is by baby formula in most stores, by baby things not by other drinks.
The point of this is to push the food through and allow the crop to empty without letting the baby dehydrate or lose too much energy, but a crop flush would be better.
I emphasize: he is very unlikely to recover if he isn't given a proper brooder. Some babies survive improper brooding, but not usually once they have already developed an issue. If at all possible please get this baby to a qualified avian vet to determine what is causing the slow crop and hopefully get it flushed.
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