My bad! It's totally legit to have 2 buttons for 2 players. I just had the reflex, as a PC player, to use the arrow keys on my keyboard. Maybe you should disable that to "force" us to play with the main buttons.
It would be great also to have the possibility to play as 2 players on a keyboard, for exemple :
Player 1 : E for the top button, D for the lower button
Player 2 : O for the top button, L for the lower button
(I avoided some keys which may change depending on the keyboard layout)