From Dragon Ball Encyclopedia, the ''Dragon Ball'' wiki
Super Saiyan Gohan and Super Saiyan Goten each prepare their own Kamehameha. Next, they fire them simultaneously so that the two energy waves merge into a larger, more powerful Kamehameha. Finally, Super Saiyan Gohan and Super Saiyan Goten charge more power into the attack to overpower their opponent, inflicting a great deal of damage.
Super Saiyan Gohan and Super Saiyan Goten used this attack in their battle against Broly in his Legendary Super Saiyan form in Dragon Ball Z: Broly: Second Coming. While they were being overpowered by Legendary Super Saiyan Broly's Omega Blaster, Super Saiyan Goten wished that Son Goku was there to help them by unknowingly using the Dragon Balls, however, Goku came back to life to help coach his sons in the energy duel, changing the Bros. Kamehameha into the Family Kamehameha (which was used to defeat Legendary Super Saiyan Broly).