I already tried pressing 8 to start the game in Two Player Mode, both above the keyboard and on the number pad, but neither worked.
Okay! My bad on that. I've put out an updated version which should just let you choose a 1 or 2 player game now while in the menus. Just hit left or right when "Start Game" is selected to choose a 2 player game.
Sorry again about that! This was stuff that was halfway in development and just got mucked up when I made a version for Itch.io.