Most vehicle mounted radar is designed to be used while patrol vehicle is moving, and can target two vehicles at once (when in fastest mode), the STRONGEST target signal and the FASTEST target signal. The unit has to be in Fastest mode, but it will display the two different speeds (as well as patrol speed) on the display.
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.