Amazingly, not only would the cancelled sequel put the two in the same game, but Vin Diesel could have reprised his Wheelman character, Milo Burik.
Unseen64 has the full info on the follow up. It would have been called Gun Runner, and focused more on cover-based co-op shooting over the original game's slo-mo mechanics.
You can get a run down of the original idea in the the video below. Midway's financial troubles eventually cancelled the game, despite an attempt to develop it without its stars, as simply 'Gun Runner'. That would have been more or less an identical game, but with lower ambitions and a more generic cast.
The most interesting part of the cancelled project, though, has to be the impressive destruction. The tech demo below shows Chow Yun-fat initially levelling the floors of a building somewhat improbably with a pistol then, later, a rocket launcher. For a PS3/360 game that's an impressive level of procedural destruction.