From the Super Mario Wiki
A Simu-Block is a block found in the Mario & Luigi series. In Mario & Luigi: Superstar Saga, this half-red, half-green variety of ! Blocks always comes in pairs and must be struck simultaneously by Mario and Luigi to be activated. These blocks are usually separated by some distance and require some puzzle solving to be able to hit them both at the same time.
In Mario & Luigi: Partners in Time, Simu-Blocks return, now in a quad-colour scheme. They appear in either pairs or sets of four, but are always next to each other. Only one of the blocks is solid at first; the other(s) are holograms. The Mario bros., their baby selves, or all four must strike them one after another to activate their effect.