This seems to be confusing people. There are two characters per kart. Not two players per kart. You will control the kart with both characters. When you turn the analog stick the driving character will stear, when you shoot the second character will use the weapons. There is only one player per kart, it will just look different. Instead of a shell appearing in the back of the kart, it will look like the second character picks one up and throws it.
One player per Kart
Two characters sit in the Kart
You dont really control the characters seperately. When you do something the driver may respond (i.e. go fast, brake, turn) or the gunner may respond (i.e. shoot shell, hit other characters, steal other items). etc.
You can play with up to 8 human players. You will have to connect multiple GCNs together, and each will need a copy of the game. You can have 8 players on two tvs and two GCNs, or use 8 tvs and 8 GCNs.