Okay so I fixed your Issue, but I found another really dumb Issue…
So Gearboxes store which sides they accepted Power from in order to not send the Power backwards. All nice and well until you realize that there is no dedicated Input and Output Sides on the thing, it just does it on the fly depending on TileEntity-Tick-Order…
As soon as you try to use 2 Custom Gearboxes to combine the Power of 4 Motors, the interactions between the 2 Gearboxes can result in one of the two Gearboxes completely ignoring that there is a face to Output to.
I really hope I can somehow remedy that because I do not think that Bug is actually fixable without making a whole Network Simulation, which would remove the simplicity of the System.