Gotenks was first sceen in the Buu Saga. The fusion is between Trunks and Goten. They had to practice the fusion dance for a while before they mastered it. Gokou didn't have much time to teach them how do do it and perfect it, because gokou had to return to the afterlife. Once Trunks and Goten mastered the dance they performed it wrong, twice, but then formed Gotenks.
