In "Game Over", when Dexter explains why he won't play "Master Computer" he mentions one of the old game's limitations is not having multiplayer, but after Dee Dee tricks him into playing with her and eventually beats him, they can both be seen playing the game at the same time, proving it has Co-Op mode. Co-Op mode is a type of multiplayer.
In "Game Over", after Dee Dee saves Dexter and removes the "Master Computer" cartridge and inserts a new one she reiterates that the deal Dexter made with her of him not getting mad if she beats him at computer games. It is then revealed the game cartridge she inserts is that of "Primal Fighter", however the title on the green cartridge has a typo spelling it "Primail" instead of "Primal".
When the episode "Game Over" starts showing Dexter and Dee a match of "Primal Fighter", Dee Dee's score increases with each successful hit, however, after she saves him from "Master Computer" and reinserts the "Primal Fighter" cartridge while Dexter is trapped inside the game console, the score remains at a constant value of 1707, regardless of how many successful hits Dee Dee has.