CHEAT MODE many lives 128, edit this cartridge 1 byte offset # $28FA, change the $04 to $7F. This is the number of reserve lives given to both players at the start. $80 won't let your first guy gain points, and higher values are negative so won't function.
The model 800 ver is a little better than this 5200 because here you can't walk off the pads while you materialize, 5200 makes you push flap button or else wait. I think they compromised to jam Joust into 16K cartridge for 5200. I'm putting better info on the model 800 Joust page.
Typo on this page says gameplay is 2 players alternating when Joust is 2 players simultaneous, and there's demo.