Change slot patterns and increase difficulty (2-players):

When you enter the two-player mode menu, hold Up and press Start. This will change the slot patterns in the game, and also increase it's difficulty.

 

Change the slot patterns:

At the title screen press B five times, then A three times, then hold Up and press Start. If done right, the patterns in the slots will be changed and the game will be harder.

 

Level select:

At the title screen press B five times, then A three times, then hold Down and press Start. If done right, you'll be able to select all levels.