I needed to be able to control 2 gears in reverse of each other for a class project. By controlling the servo, you can have the 2 gears move in equal but opposite directions.
The bottom gear has a cutout for a micro servo horn to allow for better control. You could glue these together, though I simply taped it together since the 2 components fit together snugly. If you don't want the cutout, the bottom gear is identical in size to the side gears.
Please forgive the very hacky way I affixed the gears together - using screws and bolts (yes, that's right).
You'll need supports for the casing as the gap for the servo is likely too large to bridge.