Goku and Naruto were fighting against Sasoly (Fusion between Sasuke and Broly), and they were on the verge of defeat. They figured the only way they could beat this fusion, is if they fused. They had a hard time doing so it took at least 20 minutes for Goku to teach Naruto. But, in the end they had fused and killed Sasoly. Unfortunently, they had never fused again.