It's not backup. With only one eye, humans would be already eaten by other predators, because withouth 3D vision and ability to hear well from all directions is DEADLY.
So why do not we have eye on back of the head? It would be also easier to not be surrounded :P
The sensory overload/confusion from having eyes on two opposing sides of the head might be hard to process for the brain of a human and also would be difficult to adapt to vs eyes on one side of the head.
I've decided to have juice for breakfast, the kind made out of kale, spinach, cucumbers, carrots, celery, and all that other green stuff that is hard to get enough of. To this end I bought a fancy juicer and had my first glass this morning...
it tasted like ass. It's the dietary equivalent to endurance training for a marathon, good for you, but yeah, not much fun.