Can I give a kitten meds prescribed to another kitten of mine?

No, because that wouldn't leave enough for the first kitten. Amoxi is usually prescribed for 7 to 10 days, and a bottle usually isn't much more than just enough. In any case, it needs to be discarded after 10 days. Call the vet, and see if they won't give you a new prescription for the other kitten. It's really important to give the medications as prescribed - and finish all of them.