I have thought of this: in many 3D engines, there is only one instruction to draw being called at a time. Some examples of this:
Raycaster: it only draws one sprite at a time. One "guy" to do all the work.
Mode7: one drawing instruction is called in a loop. Only one "guy" to render the whole screen. To fix this, make it render 2 times in a loop or more.One renders the left, the other doing the right