1P 2P? What kind of character name is that?
Pop n' Pop's Story Mode allows for two-player co-op. If both players pick the same character, that pair's intro will play.
Select two different characters- like Bibby and Chack'n- and a different intro will play- a more generic one.
(The intro was later reused for the Game Boy Color port of the game, for every character).
Custom Co-Op - Story Mode Intro
Custom Co-Op - Story Mode Ending
Incidentally, this set-up is the only way to see Dreg's back-sprite, as he never has the 'meeting Lena' scene in his ending.