It's a very, very difficult one to give a single and simple answer too.
If we take the simplified explanation that the accident is 100% the pedestrian's fault, and that absolutely no fault could be attributed to the driver who initially strikes him, then the insurer of the second driver should (I think) be taking action solely against the pedestrian.
The same would apply if a vehicle pulled out of a side road, was hit by an oncomging vehicle, and was pushed into a third, parked, vehicle.
However, in real life, the scenario you put forward would be an absolute nightmare to deal with.
I've personally seen one where a pedestrian has had a form of epilepsy, had a fit, and in the trance he was in, has run out of his house and into the path of our policyholder. He got up, then ran into the path of a bus. He has then somehow ran to the top of a caqr park (multistory) and thrown himself off the roof.
The fit caused him to go into some sort of trance where he wasn;t in control of his actions, but as he lay unconcous for a while, there was much buttock clenching as to how much damage our initial impact had caused as he had multiple injuries, any of which could be attributed to our impact, the impact with the bus, or jumping off a building!
All sorts of factors were taken into account such as whther the house he ran out of had a front garden (so our policyholder should ahve seen him before he ran into the road), etc. very complicated