To control both vehicles in one player mode simply highlight the two player option on the title screen and then press Start and A.

To do a level skip simply pause game play and enter one of the following codes while playing in the corresponding level.

From level 1 to level 2
Press R, L, R, R, L, L, R, L, R, R, L, L.

From level 2 to level 3
Press R, L, R, R, L, L, R, R, R, L, R, R.

From level 3 to level 4
Press L, R, L, L, R, R, L, L, L, L, R, R, R, L.

From level 4 to level 5
Press R, R, R, L, L, L, R, R, L, L, R, L, R, L.

From level 5 to level 6
Press R, L, R, L, L, L, R, R, L, L, R, L, R, L, L.

To return to the title screen simply press Start and then hold Select.