The one who applied most damage prior to the breakage of any 'critical' part does get the kill credit. Doing the critical damage itself does not matter. It just stops any further damage counting towards kill crediting.
For example one fighter pilot rakes a bomber all over with his .50 cals, but doesn't actually break anything. Then another one comes in and does a last tiny bit of damage to remove a wing.
The former player will get the kill.