This project is a modification of the Arduboy2 and ArduboyTones made to be used with an Arduino Mega 2560.
The second players' buttons can be accessed in code as LEFT_BUTTON_2, RIGHT_BUTTON_2, UP_BUTTON_2, DOWN_BUTTON_2, A_BUTTON_2, and B_BUTTON_2.
See more details on this project at hackster.io.