it could be 1 of 20 things, really impossible to tell by your description.
here is just a short list of what could be wrong
*crop infection
*impacted crop
*punctured crop
*bacterial infection
*intestinal bacterial infection
*bowel obstruction
*dehydrated
*failure to thrive
*maybe it digested something it shouldn't
the list can go on and on.
I would separate the "sick" one from the other baby for now, if it's contagious, you don't want both to get it, other than that I would seek a vet.
I had two babies that were 2 weeks old with stunted growth, failure to thrive, along with crop infections, I took both to the vets,gave them anti-fungal meds along with an antibiotic, they died 2 weeks later.