I think Josh makes a good point about keeping the humidity at a level that would be acomidating for the chameleons but also hight enough for the mantellas. This would have to be monitered and figured out.
A full grown Mantella should be quite the job for a chameleon to tackle but i would say there is always a chance they might try to snap at them. But keeping them well fed with smaller more managable prey I would expect lower chances of that happening greatly.
I found this on another webpage on Brookesia. "Other Malagasy species with similar requirements can be housed with Brookesia in this sort of vivarium, provided there is sufficient space. Mantellas can add a splash of color, reed frogs (Heterixalis sp.) can add some nighttime activity, rain frogs (Scaphiophryne sp.) make interesting but seldom seen additions, small Uroplatus and Phelsuma can also be added." http://www.chameleonnews.com/year2002/s ... kesia.html
That site has some good info