The general rule of thumb, like Octopus said, is to separate them. They are opportunistic predators who will try to eat nearly anything, even if it is physically impossible. That isn't to say that they can't be kept together, but typically it would only be for short periods of time for things like breeding. The other benefit of separation is that when together, they produce twice as much waste as if they were separate, so you have to really clean quite often. That being said, it really is up to you. If they're the same size, are fed regularly, and the cage is cleaned often, you may be ok.

Alex