Co-Star Super Jump
From the Super Mario Wiki
The Co-Star Super Jump is a jump that Mario or Luigi can perform in Super Mario Galaxy, though only during Co-Star Mode. To perform this jump, a second player needs to aim at Mario or Luigi with the Star Cursor and press the button at the same time as the first player. The jump will make Mario or Luigi go higher than normal, allowing him to reach items or objects usually out of reach. If it is done at the wrong time, Mario or Luigi will just jump normally. Mario or Luigi can still control the direction of his jump when performing this move. This trick even works in midair, but only works once before touching a horizontal surface. This move cannot be performed in Super Mario Galaxy 2.