From Dragon Ball Encyclopedia, the ''Dragon Ball'' wiki
The Multiple Kamehameha (マルチプル かめはめ波, Maruchipuru Kamehameha; Literally meaning "Turtle Destruction Wave Multiple") is an energy wave attack used by many fighters.
Two or more fighters fire multiple Kamehameha (one per fighter) simultaneously at a single point. This usually causes their separate Kamehameha to merge into one giant Kamehameha against theit opponent.
Dragon Ball Z: The World's Strongest
Dragon Ball Z: The Tree of Might
It was also used in Dragon Ball Z: The Tree of Might where Goku, Kuririn, and Yamucha used their Kamehameha along with Tenshinhan's Tri-Beam and Chaozu's Dodon Ray to try to blow up the Tree of Might, but to no avail.
Dragon Ball Z: Lord Slug
Dragon Ball Z: Broly: Second Coming
This attack was later used by Son Gohan and Son Goten along with the spirit of their father to defeat Broly in Dragon Ball Z: Broly: Second Coming. Goten, Trunks Brief, and Kuririn also used the Multiple Kamehameha to cause a tidal wave to overflow the laboratory island and solidify the Culture Fluid that was quickly expanding all over Jaguar's laboratory island.