The Axle might make this work better, i just noticed that your issue is actually that the gearbox to the left is inputting into the custom boxes, while the one on the right is at the “output” side of those gearboxes and therefore not accepting input (it might be that i messed up left and right, its a timing thing not a direction thing).
Try starting all motors at the exact same tick and it should work with your screenshotted setup. Sorry for the inconvenience. ^^"