The central axis allows the two twist controls to rotate independently whilst still being on the same axis. It consists of a hollow cylinder that houses 4 bearings which are attached to two separate rods. The cylinder holds the two rods inline and on the same axis, but separated slightly in the middle allowing, in conjunction with the bearings, the rods to rotate individually.
To create the cylinder, sand the corners of the piece of pine to create a cylinder of dimeter 44mm x 45mm. Using a pillar drill, drill a hole of dimeter 28mm all the way through and slot in the 4 bearings. Feed the two rods in, through the bearing at either end, adjusting accordingly should they touch in the middle.