In Soviet Russia, game plays you!
C-Spec would be telling your race engineer how to instruct his driver. He would even choose not to listen to you sometimes and tell his driver to do something else and his driver wouldn't even listen to him either sometimes, making it an extremely fun scenario that isn't at all frustrating.
Combo Spec. You and Bob driving together.
This would be an ace idea for the A-spec endurance races - I'd grind out a few hours, then hand over to one of the Bobs for a while.
C-Spec is where you become Bob's Bob.
He tells you how to race.
And you constantly hold off going flat out... when's he's screaming at his monitor telling you to floor it!