Peter,
You need to add the section plane INSIDE the car_A component. Oriented to the front.
INSIDE car_B component you place a second section plane facing to the rear end of the car.
Both section planes can be active at the same time for they are in different components.
The rest is up to you.
Wo3Dan
p.s. Toggle off "Display Section Planes" and have "Display Section Cuts" ON