Two player mode is a bit weird, yeah. To activate it, set up the game however you want to play it, and then instead of hitting the Enter key to start the game, press 8 instead. There's a very specific and weird reason behind this, but I won't go into it unless you want.
To delete high scores, you'll have to locate the INI for the game. It can be found in:
C:\Users\(your name)\AppData\Roaming\MMFApplications
It'll be the file called "73mp.ini". You can open it in your notepad app. Look for the lines that say "PacHighScore" or similar and delete them, then save the INI. Or you can just delete the whole INI as well, but that will reset all your settings too.