Sounds like the start of a elaborate motorsport joke, but WIlliams actually allowed multi-time 500cc champion Mick Doohan and multi-time WRC champion Tommi Makkinen into their F1 car in the summer of 1998, all decked out in Winfield red:

Although they received "expert" training from reigning F1 champion Jacques Villenueve (which seemed to be having a chat while trundling around the circuit) BOTH drivers ended up costing the team several thousand pounds worth of damage!