First, Goku and Pan each prepare their own Kamehameha attacks. Then, they both fire them at the same time so that the two energy waves merge into one giant Kamehameha blast at the opponent, inflicting a massive amount of damage.
Goku and Pan use this attack to defeat the Shadow Dragon Haze Shenron. After getting thrown into the polluted lake by Haze Shenron, Goku and Pan are on the verge of dying until Giru rescues the two and brings them to a fresh water geyser. Goku blows up the geyser with a Kamehameha, allowing the fresh water to clear the lake, much to Haze Shenron's horror. Goku and Pan regain their strength and fly up to confront Haze Shenron, who frantically attempts to escape down to the lake. However, just as Haze Shenron flies down to the lake, Goku and Pan fire the Grandfather-Granddaughter Kamehameha down at Haze Shenron, destroying the Shadow Dragon. With Haze Shenron's death, the polluted air dissipates and the fresh air is restored, allowing Goku and Pan to recover the Two-Star Dragon Ball.