I would keep them in separate tanks. The Kaloula pulchra eats small prey, like small runners, small crickets or wood lice (in the wild they eat termites and ants). The tomato frog, be it real or false, eats larger prey, at least mine do, like large roaches, pinkie mice and so on.
The tomato frog will be stressed out by the kaloula and vice versa, and the tomato frog might eat all the food the kaloula is soppused to eat, and so on.