According to the ATSB video :
Viper 2 was planned/briefed to stay to one side and clear of viper 1 during any manoeuvres and for the shot.
Viper 2 did the fly under manoeuvre unbriefed the first time.
The safety pilot on V1 pointed this out to the pilot who then had a discussion with Viper 2 who explained that that manoeuvre provided a better shot and they agreed to try it again. (Anyone want to bet ‘what the f#ck is he doing’ was muttered by someone on V1?)
During the second attempt they had the impact.