Some barks contain oils that can be harmful to your chameleon...but if the same bark has always been in both cages, unless the little one did more walking around on the bark, it shouldn't have killed one and not the other.
There are some lights that cause health issues. What are the brands and types (spiral, compact, etc.) of lights you have on the little one's cage?
If the divider was not up the whole side of the large cage and the little one could see the bigger on he could have been scared to death.
As was suggested...a necropsy might give you answers.
There are some lights that cause health issues. What are the brands and types (spiral, compact, etc.) of lights you have on the little one's cage?
If the divider was not up the whole side of the large cage and the little one could see the bigger on he could have been scared to death.
As was suggested...a necropsy might give you answers.