From Dragon Ball Encyclopedia, the ''Dragon Ball'' wiki
The Grandfather-Granddaughter Kamehameha (祖父 孫娘 亀破滅波, Sofu Magomusume Kamehameha; Literally meaning "Grandfather Granddaughter Turtle Destruction Wave") is an energy wave attack used by Son Goku and Son Pan. Goku and Pan each prepare their own Kamehameha. They both then fire them at the same time so that the two energy waves merge into one giant Kamehameha that blasts their opponent, inflicting a massive amount of damage.
Goku and Pan used this attack to defeat Haze Shen Long in "The Two-Star Dragon". After getting thrown into the polluted lake by Haze, Goku and Pan were on the verge of dying until Gill rescued the two and brought them to a fresh water geyser. Goku blew up the geyser with a Kamehameha, allowing the fresh water to clear the lake, much to Haze's horror. Goku and Pan regained their strength and flew up to confront Haze, who frantically attempted to escape down to the lake. However, just as Haze flew down to the lake, Goku and Pan fire the Grandfather-Granddaughter Kamehameha down at Haze, destroying the Evil Dragon. With Haze's death, the polluted air dissipated and the fresh air was restored, allowing Goku and Pan to recover the Two-Star Ball.