[JasonB]

Here is a bit from the DS200 Manual, which might be relevant. It sounds as though one of the sensors is simply ignored in rally mode.

Specific race for RALLY-SLOT:
The race is intended for 1 car only. The racer can start
when he wants, indicated with “----“ in the displays. After
the green light is on, when the car crosses the sensor the
clock is activated and timing starts, this allows the correct
running of the car to be checked before the race starts.
LAP TIMING: Lap timing is shown on display 2 each lap.
RACE TIMING: You can always see the race time in display1.
At the end of the race, the seconds elapsed are shown in
display 1 meanwhile the 1/10th, 1/100th and 1/1000th of
a second in display 2.