Double Attack is a team attack used by Goku and Piccolo. It starts with Goku charging at the opponent, just when he is about to attack, Goku teleports away, leaving the opponent open for Piccolo, who rushes in and lands a barrage of punches and kicks. With the last kick flying him over to Goku who lands the final blow, making the ground shake as the opponent crashes to the ground.
Double Attack is exclusive to the Nintendo DS game Dragon Ball Z: Supersonic Warriors 2. It was first used in the game when Goku and Piccolo faced Vegeta, and used this to defeat him, but it did not work and Vegeta killed Piccolo (replacing Nappa's role in the story).
If Vegeta is in the team at the time, a different, more powerful attack is done by the three of them. This version is finished with them firing the Galick Gun, Special Beam Cannon and Kamehameha at the opponent.