Normally, a ride-along is in one vehicle. The idea should be to get to know the employee better; work beside him/her, find any weak spots etc.
The one caveat I feel I must add is if the Supervisor/Manager needs to break away during the day to another commitment. This, ( in my humble opinion), would be a classis example of poor planning. If you're gonna do the 'ride-along', clear your calender!
The other example of a 'ride-along' is for cross-training; Dispatchers, management, HR, etc. Should always be in the same vehicle; hence the term.........'Ride-along'.