I could imagine a lot of stress with different species being together. I'm sure there are exceptions, but I would go with a "don't do it" answer.
When I take Yuki out of her terrarium, which is next to Marimo's (They can't see each other while in them), he makes himself look larger, and shows a very aggressive attitude towards her. If I bring her nearer, he walks away, always looking back. She looks at him as if he would be a light snack after dinner...
This might be because she is far bigger than he and really COULD eat him if she wanted. And no, I don't put them near each other anymore. One time thing for a second already gave me my answer!
That being said, I will and wouldn't suggest ever mixing chameleons. I can't speak for pygmies or others who might have had success, but I wouldn't risk it. MAYBE if you have a HUGE free range...it COULD work.
While it is not advised for the same reasons that even same species chameleons should not be housed together, there is also the added risk of exposing one species to the pathogens of the other since they are potentially from different continents or evolutionary background. Look up herpesvirus in elephants for more info on this type of problem. How different can Asian and African elephants be when they look so similar? Different enough that a herpesvirus that is harmless in Asian elephants will outright kill African elephants when they're exposed to it. There are similar viral crossovers causing death when some tortoise species are mixed. There is always the potential for a simar scenario when mixing species.
Amen Ferret! Why do ppl always seem to want to put two or more chams in one enclosure? They arent dogs, chams arent pack animals they dont like to interact with each other except to breed. And even then it seems to be more of a mutual short term acceptance than anything.