To say it more precisely, you can connect two opposite sides of a gearbox by two ways. The one you used: placing 3 interlocked gears. And a second one: by placing no gears and just using a monkey wrench on one of the sides to put an internal axle through the gearbox. These two options offer different connections, the first one is good for connecting two sources where each is rotating in a different direction, and the second one when they are rotating in the same direction. After you put the internal axle through the gearbox you can also transfer the power to the side by placing two interlocked gears on it as well.
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