Very simple, when they are adults if the tank is huge the RTC wouldn't be at the same level as the ray's it would be slightly above that is why it is possible to see them together.
But the real problem is when they are both small, with no doubt the RTC would grow faster than the ray and the tank wouldn't probably be big enough and he would heat her, she might sting him and they might both die.
In my opinion I would never mix, but in my opinion too, who has a big enough tank for a 1.5m RTC?
I also don't support people keeping TSN for the same reason, although I love monsters, some fish are not aquarium fit..
Buy fish like Tigrinus and Juruenses thay are much more worth it.
But this is just my opinion