Even if they are close to the same size one might try to eat the other, as mentioned above they are not the brightest frog. And sometimes it wont just be damage that is caused, you could have a chance of losing both frogs. Due to the little teeth they have in front on the bottom of there mouth sometimes if the item is to big they cant push it out and the one swallowing can choke and the other frog could die from the injuries he gets from being bit.