Firstly, what are the species are both your chameleon and your tortoise?
That being said, no. Do no mix them. Mixing species is NOT a good idea, ever. The bacteria changes from animal to animal and they could make each other sick. Also, being together would only cause stress. Reptiles are not like we are...they don't enjoy interaction with members from different species.
Plus, tortoises (most) need more air flow and should have an enclosure with an open top to allow this, not an enclosed viv.
You could, but why would you want to! Each species has it's own pathogens, and when you put two different species together you are looking for trouble. I am assuming you meant chameleon. And I'll confess to not really knowing the answer, but my experience tells me it's not a good idea. But lets wait and see if a more experienced keeper chimes in...
Chameleons are one of, if not the most easily stressed reptiles commonly kept in captivity. Even though the tort would be on the ground and the cham would 'up' most of the time it's just not a good idea at all. As the others said, the risk of disease is a concern as well. Most chams are pretty fragile really, it's not worth chancing in my opinion.
Chameleons are not reptiles that do good with any reptiles. They dont do good with other chameleons and i'm sure not with a tortoise. Plus Chameleons like tall enclosures, vs a tortoise which like wide and lots of floor space. Pretty much even if they both were from the same area and could live with each other, it would be hard to give them both comfortable environments.