From Dragon Ball Encyclopedia
First, Super Saiyan Gohan fires a Super Kamehameha at their opponent, who struggles against the attack. Next, the spirit of his father, Son Goku, appears behind him in a Kamehameha stance and shouts to him, "Now!" Finally, Super Saiyan Gohan pulls his hand back and throws it forward as he charges more power into the attack, overpowering their opponent and inflicting a massive amount of damage.
Dragon Ball Z
Super Saiyan Gohan used this attack in his final battle against Super Perfect Cell. Though Super Saiyan Gohan managed to combat Super Perfect Cell in an Energy Struggle, the Artificial Human nearly overpowered the young Saiyan several times due to having his arm broken when protecting Vegeta from Super Perfect Cell's attack. As Super Perfect Cell prepared to finish off Gohan, however, Vegeta blasted Super Perfect Cell with a Big Bang Attack and completely distracted him. Super Saiyan Gohan saw a chance and was encouraged by Goku. Super Saiyan Gohan unleashed all of the rest of his power into the Father-Son Kamehameha. He overpowered the Solar Kamehameha and destroyed Super Perfect Cell once and for all.
Father-Son Kamehameha appeared in the Budokai video game series and the Budokai Tenkaichi video game series and is Teen Gohan's Ultimate Blast in his Super Saiyan 2 form. During the epilogue of the What If Saga The True Ultimate Android in Dragon Ball Z: Budokai Tenkaichi, Future Cell and a Cell Junior used this attack instead to overpower Gohan and defeat him.
It later appeared in Dragon Ball: Raging Blast where the color was changed to bluish-green.
Another version appeared in Dragon Ball Z: Supersonic Warriors 2. In this game, Goku and Gohan used the technique, but their opponent blocked it. Suddenly, Bardock appeared and blasted their opponent, knocking them off guard and allowing them to be overwhelmed by the attack.