Depends entirely on the road situation. 2 abreast vs single file really doesn't make much difference to a car as bikes are generally a LOT slower than cars are. If you can't overtake a bike, or two bikes whether in single file or 2 abreast, safely in a car you really shouldn't be allowed on the road.
Riding 2 abreast is good for 2 things:
Making cars slow and consider the overtake.
Making cars angry.
Riding single file is good for 2 things:
Stretching out the length the car has to pass
Making it less likely that the car struggles to pass by allowing them some room to make a mistake.
At the end of the day there's a right and wrong for every situation, and for every person in that situation, but they rarely are the same for all people in that situation (as seen in this thread).