Hi Dharma,
There are no hard and fast rules regarding fault, especially in unusual situations such as this. However, generally if only two cars are involved in an accident, the car that hit from behind is liable. If two cars are reversing from opposite car parks, both parties will be equally liable for damage. If one car had stopped moving and was stationary at the point of impact, then the other car continued to reverse - the latter is liable for the damage.
Therefore, I would say if they were both still moving at the point of impact they are both liable and should share costs. If one of them was stopped at the time the incident occurred the car that reversed or drove into the other is liable.