The "two mystery jocks" refers to two jocks that appear in a cutscene during the mission The Gym is Burning.
Both Jocks wear wrestling uniforms.The white mystery jock is a character named Bob, who was removed from the final version of the game (excluding this cutscene) for unknown reasons. His name was determined from the game data files. During the cutscene Jimmy insults him by saying he "has the personality of a jock strap" but doesn't call him by name.
The other Jock is Bo Jackson. In the cutscene, his voice is somewhat higher pitched than Bo's usual speaking voice, and he has a different hair style (Bo's hair, which can be seen if his cap is knocked off, is round rather than a flat top). However, the facial features are an exact match.
Both names were verified once the game data files were cracked. Bob can be hex edited into the game with his wrestling model.