Okay, this should simplify the design quite a bit.
1) Pick any gear size that works for your design.
2) Place two holes along the arm's axis an equal distance from the gear's center. These will be used for your two sided drive. (The sum of the distances between the top holes and the bottom holes is a constant making it perfect for a closed circuit.)
I used herringbone gears for axial and rotational alignment. I offset the herringbone gears from side to side by 1/2 of a tooth. This allows you to use the same part for both sides of the design. You could use string or nylon tip ties to hold this together. It would be easy to add tension from the back of the arm. Additionally, this looks very printable. I made the helical angle 45 degrees to aid in the printability.