It is my understanding that once a plane is "doomed".. in other words crashing or exploding is inevitable.. then any further hits on that plane do not register.
For example.. player A dives on a plane and takes its wing off... as the enemy is banking over due to the sudden loss of lift on one side player B attacks the plane getting several hits and maybe even blowing it up. Since the plane was already doomed when player B attacked it, those hits do not count and player B does not get an assist.
It is also my understanding that the kill is awarded not to whoever hit it first, but to who did the most damage. Example.. player A dives on a plane and hits its right wing four times with a 20mm gun, putting several holes in the wing but not destroying it. Player B then dives in and hits the same wing with two rounds of a 20mm gun causing the wing to rip off and the plane to crash into the ground. In this example player A would get the kill because they put more ammo into the plane even though player B actually destroyed the wing.
