You use more than one Driver in races that allow more than one driver.
They will then swap out when you pit stop.
The first races to allow 2 drivers are the European races in extreme level.
Then Super GT, Japan events. (This championship is the first long series for bobs . some races can take nearly 1 hour so you will have to pit stop for fuel normaly...)
Then up to 4 drivers in endurance races....
You just drive them.
Mood arrows are not to do with tiredness the performance dip is yet again another randomiser which you must allow for, being the manager of the team, they will go back to Blue on their own but instead of you wondering why they suck more, there is this visual system to tell you to expect them to be worse...