Double Item Boxes are a feature introduced in Mario Kart: Double Dash!! and Mario Kart 8 Deluxe, being the only games with this feature. When a duo's kart collides with these, both partners will receive an item; if one already has an item, however, only one will be received. These may randomly replace normal Item Boxes on the normal paths, but will always appear in special shortcuts.
Like normal Item Boxes, Double Item Boxes give you items. But not just one, rather two; one for each partner. When received in single-player mode, the back partner uses his/her item, then switches with the driver who then uses his/her item. However, in multiplayer mode, the driver must pass his/her item to the back partner who throws the item for the driver. Double Item Boxes also returns in Mario Kart 8 Deluxe with the same function as before.