Should the officer lock the speed (on the device) on the first vehicle, before proceeding to target the vehicle behind the first vehicle?
If the Officer doesn't lock the speed, are they able to determine what the speed is of other approaching vehicles?
Is it possible for an Officer, who is mobile and targeting approaching traffic, to pull over 3 - 4 vehicles at one time?
No they do not need to LOCK in any speed to get both readings.
The radar does NOT tell the officer which vehicle is the fastest and which is the strongest, so the officer has to make this determination by what they see. If there are more than two vehicles on the road, then it would be very hard to pick out which vehicle is giving which reading as a vehicle further away could be going faster than multiple closer vehicles.
A misconception is that the bigger object (like a truck) will have a stronger signal than a smaller object (like a car), but this is only true if they are side by side. If a smaller object is closer to the radar than a larger object, the closer object will have the stronger signal.
If one officer has multiple vehicles pulled over, then most likely there is another officer in another stationary vehicle with the radar telling the second officer which vehicles to pull over.