The gearboxes seem to be working much better, however, I’m still having some issues applying torque to the axle via two gearboxes. See images below (the right motor is not connected).
As soon as I connect the left motor to the axle in the back gearbox via the axle of the front gearbox (order of inserted gears and axle doesn’t matter), the system halts. (I tried both rotation directions for the left motor)
However, if I set the left motor’s rotation direction appropriately and use gears to connect the front gearbox to the back gearbox’s axle, the system works as expected.