As you can see in this picture from the Texas during World War II, carrying two on the catapult was standard practise. (Seen if you click on the picture for the full size and look at the catapoult area…both planes are on there)
The reason was simple. They vessel normally carried two Kingfisher Scout aircraft. They only had one catapult. So when not in use, they would keep them attached on the catapult…stoing them there. When needing to launch, they would remove on aircraft with the crane and hold it until the other aircraft was launched. Then, they would place the other on the catapult ready to launch.