In addition, bettas need a certain amount of swimming space. A bowl is too small for two bettas to swim comfortably. This can lead to stress, which can make the fish more susceptible to disease.
If you want to keep multiple bettas, you should get a tank that is at least 10 gallons. This will give the fish enough space to swim and hide. You should also provide plenty of plants and decorations so the fish can feel secure.