It's not possible to have two people playing simultaneously on one keyboard.
However, you can pass a keyboard (or controller) between two or more players to take turns playing by using the Shared Controller mode. This only works in Local play.
In the Treehouse, go in front of the red couch and press the Jump button.
This will allow you to select an additional character as P2.
When you start a game, you will be in Creative mode. Each player will take their turn to place blocks on the level, and then each will get to run through the course.
It's not possible to play in modes other than Creative when you're playing with a shared controller.