One Cage- Different Reptiles

[Gezt]

I know this is a big no no, but a recent trip to the Toronto Zoo had me thinking. They had quite a few reptiles in shared cages. One was a Green Water Dragon with a Box Turtle, and I do remember a Chuckwalla (I think) and some other smaller lizard (I think it had spiny in it's name...). Well, anywhoo, if a large enough cage is provided that suits all animals, why is shared caging not used more often? Also, a list of reptiles that can co-exist would be awesome!