Summary: It makes no sense for a tangent driver to follow the Saturn from where it started, if we assume that the Saturn's driver could fit in the tangent driver's car when the drive began. That is, unless the theoretical tangent driver needed to return at a different time than Maura did. Therefore, if there was a tangent driver, and if the theoretical tangent driver didn't need to return at different time than Maura, the tangent driver's starting point must have been far enough away to justify the use of both vehicles.
The further apart two objects are from each other, the less likely they will cross paths.
Summary: The greater distance two tangent vehicles commence their journey, the less likely they will arrive at their intended destination if one of the two vehicles is interrupted accidentally. Especially if the drivers are unable to communicate with each to correct for the accidental interruption.
The two principles are inherently at odds with each other. Distance justifies a tangent driver. But distance makes the likelihood of the vehicles meeting unlikely, given that we know:
1. The Saturn was in an unplanned accident
2. The Saturn's driver had no way to communicate with a presumed tangent driver
The idea that the drivers of the vehicles could overcome the disruption to their drive and therefore successfully meet, is at odds with "The Law of Least Luck" linked here:
Note: That if Maura Murray was not the Saturn's driver, a tangent driver makes sense, only to the extent that the driver intended to dispose of the Saturn purposely. However, it is not reasonable to assume that if the Saturn were being disposed of purposely, that the driver would have gone to such distance to do so.
On the other hand, if the Saturn's driver knew they could be misidentified as Maura Murray (per chance the Saturn's driver was seen), driving the car that distance certainly caused significant misdirection in this case.
Note: The evidence weighs heavily against the prospect that Maura Murray was not in the Saturn at the accident site.
A poorly functioning vehicle at rest, tends to stay at rest; a well functioning vehicle in motion, tends to stay in motion.
Carpoolers in motion tend to occupy well functioning vehicles in motion, unless occupancy capacity is exhausted.
Carpoolers in motion tend to occupy poorly functioning vehicles in motion, when well functioning vehicles in motion lack the capacity to accommodate them.
Carpoolers at rest, that occupy poorly functioning vehicles at rest, tend to stay at rest.
A vehicle in motion tends to reach its intended destination.
A vehicle at rest tends to not reach its desired destination.
A cell phone out of service tends to stay out of service. It cannot be used to instruct a vehicle in motion to adjust its course.
The major legitimate objection to the idea that a Tandem driver was unnecessary, has been mentioned a few times before. I have found one specific example of a statement made by /u/mulan3237: "I think a tandem driver makes sense if we think Maura was only intended to get away for a week or so and someone was going with her for just the first few days and they took their own car to drive back separately."
However, this explanation does not address how the theoretical tangent driver would have known Maura had an accident.
One might assume that the theoretical tangent driver was following Maura. However, since this theoretical driver was not seen by the Westmans or Butch Atwood until at least 4 minutes after Maura's crash, it is clear that no car was following close behind.
A tangent driver would not follow the Saturn closely and then decide to hide from Butch Atwood without apparent pressing reason. Furthermore, a theoretical tangent driver would, presumably, be unfamiliar with the area, and therefore, would not know where to hide until Butch Atwood left.
Simply put, it would be bizarre for a tangent driver, prior to Maura's disappearance, to feel it important to keep Maura's trip a secret. If there was a tangent driver close by, that individual would have pulled up while Butch Atwood was talking to Maura and would have been seen giving Maura a ride.
We have no evidence that Maura Murray was in the kind of danger that would require the drastic measure of mysteriously exiting her life, a mystery that remains unresolved almost 14 years later. Perhaps a motive could exist, if, Maura was harmed after being picked up by a theoretical tangent driver, but certainly not before.
What this tells us is that there could not have been a tangent driver close enough to the accident, such as to allow for Maura to be picked up in the short time window of 4 minutes, if we assume that the vehicle that arrived at around 7:35, picked Maura up.