Mario Kart: Double Dash!! (note the two exclamation points, representing how Nintendo played pun perfectly) is essentially Mario Kart 64, except you can have two drivers at once, there's tons of new tracks and items, it's on the GameCube, and it's nothing like Mario Kart 64 at all. It also introduced Toadette and included a few baby characters.
A demo of Mario Kart: Double Dash!! was released on Interactive Multi-Game Demo Disc Version 13. There's a bunch of unused graphics and differences hidden in this demo.
It seems like a Reverse Cup was planned, making the player race the tracks backwards. This also indicates that in LANPlay.arc, Yoshi Circuit, Dino Dino Jungle, Peach Beach, and Mushroom Bridge (coname_yoshi_circuit_r, coname_dino_dino_jungle_r, coname_peach_beach_r, and coname_kinoko_bridge_r) have text for it in Japanese. The "r" most likely meaning "Reverse". This would eventually become a reality in Mario Kart Tour.