No, it's not turning two inputs into one output.
The only input is from the left gear, which is attached to the motor. The output is via the right-hand gear, which goes to the focus mechanism.
The only purpose of the middle gear, the idler, is to transfer power from the left gear to the right gear...but only when the linear actuator (or whatever) has moved the motor to the right, so that the left gear engages the idler.
When the motor is moved to the left, so that the left gear is NOT engaged, you will be able to turn the focus mechanism by hand, which will rotate the right gear and idler, but not the motor...
If you use the same size of left and right gears, the output rotation will match the input rotation (but be opposite direction). You could use a smaller idler (middle gear) than the one shown...its size has no effect on input vs output.
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