When performing it, Gohan and Goten each prepare their Kamehameha attacks. Then, they fire them simultaneously so that the two energy waves merge into one large Kamehameha blast. When this is not enough to defeat the opponent, Gohan and Goten put more power into the attack, overpowering the opponent and inflicting a great deal of damage.
Gohan and Goten use this attack to combat the Legendary Super Saiyan Broly's Omega Blaster in the movie Dragon Ball Z: Broly - Second Coming. However, when Goten wishes that Goku is there to help them by unknowingly using the Dragon Balls, Goku comes back to life to help coach his sons in the energy duel, changing the Bros. Kamehameha into the Family Kamehameha (which is used to defeat Broly).
In Dragon Ball GT, after Gohan and Goten are infected by the Tuffle parasites created by Baby, the Saiyans destroy Grandpa Gohan's house and try to eliminate Goku and Pan using two energy waves which are fired against they from ground to the sky. This double attack is considerably powerful, but he can only partially stun Goku and Pan with a reverberant blast.
Video Game Appearances
In Dragon Ball Heroes: Ultimate Mission X, Frieza and Cooler are capable of performing the Bros. Kamehameha.
The Bros. Kamehameha appears in Dragon Ball FighterZ as Adult Gohan's Meteor Super Special Attack, but the move will change depending on team composition. If Goku is on Gohan's team the attack will become the Family Kamehameha, but if Gotenks is on the team Gohan will perform the Ultimate Kamehameha instead (because Goten isn't able to assist). However during the main story, Gohan will also perform the Ultimate Kamehameha instead regardless of the team composition. Presumably this is due to portions of the story where Goten is absent such as before the Z Fighters manage to locate Goten and Trunks.