Do not house your panther chameleons together. It may seem to you like they enjoy it, but in reality they do not. This should become obvious over time. I often see this in pet stores with multiple sub-adults in the same enclosure; one chameleon will look and behave somewhat healthy, and the other(s) will slowly die off from stress and malnutrition. How long have you been housing them together? How old are they? What size enclosure?
Are panthers less likely to breed if housed together year round?
Unless you’re some sort of chameleon mind reader then you don’t know that. Signs of stress can be subtle with no visual cues at all. As mentioned, given time, one of them (most likely the female) will end up getting sick.