A differential consists of two axles connected by three cogwheels. If one axle is turned in one direction, the other one turns the other way.
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If both axles are turned the same way, the box holding the cogwheels must turn with the axles. If the speed of both axles is not the same, e.g., if one axle stops and only the other axle turns, the box has a different speed and the center cogwheel turns to compensate the difference in speeds.
If two differentials are connected, each of the three axles can turn at a different speed. In particular, if the center axle rotates and the exterior axles do not, the boxes will turn in the same direction.
If the center axle is fixed and the exterior axles rotate in opposite directions, the boxes will rotate in opposite directions.
This vehicle applies the top motor to the center axle and the bottom motor to the exterior axles, with an odd number of cogwheels to reverse the direction. The wheels are connected to the boxes. In this fashion each motor drives both wheels, one motor moves, the other spins the vehicle. A single rotation sensor can measure both, movement and turning angle.