Hi DSC,
Mutual flying will be a moot point here I'm afraid, they would of both known it had gone pear shaped long before "terra firma" jumped up to grab them and open the pearly gates.... and I'm sure they would of both been crystal clear to each other as to actions and inputs to try and recover the situation.
The aircraft would only have entered an erect flat spin for one of two reasons, the first being intentional spinning that either they couldn't recover from due to weight and balance or incorrect recovery technique as well as insufficient height to recover from, or a high energy aerobatic manoeuvre that resulted in a flick to spin resulting in either disorientation of direction of spin rotation or again incorrect recovery technique, along with height to recover.
This is a terribly sad, bad flying accident with two young souls that were obviously keen enough to go and hire an aircraft whilst serving in the RAF undergoing flying training.. whatever they were doing and trying to do or practice, it went wrong and they paid the ultimate price... It was an accident and a terrible one... Lessons will come out for sure, but with no CVR or data recorder it will not be exact it will be approximate - either way we will all learn and adapt...