There are several reasons not to keep more than one cham in the same cage. First, most cages pet owners are using are simply too small to permit more than one cham to bask, drink, eat, sleep, and establish its territory without being constantly stressed by the presence of the other. In the wild the animals have the ability to move away from each other. They don't in a cage. Even though they may not actually fight, they do show aggression visually by posture and color change. Even seeing each other all the time from across a room could be too much stress. Their primary sense is sight after all.
Second, once the male is mature he'll be interested in breeding regardless if the female happens to be receptive. She can't get away from him and ends up stressed, harassed, possibly injured, or she could injure the male trying to defend herself. If they do breed you'll need to separate them anyway as the female will not tolerate the male once she is gravid.
Third, every cham is an individual. Some chams are more tolerant (they have smaller personal territory) than others. By the time the keeper knows their particular cham personalities and space needs it may be too late...one cham is already in trouble, sick or dying. Why take that chance?
The space needed to keep more than one cham together varies and the individual experiences of different keepers make it hard to be precise. I would not keep two veiled chams in anything smaller than a bedroom.
