I'm normally loathe to put forward theories, but in an effort to dispel the thinly veiled allegations of unauthorised formation flying, I'll expand slightly on JugglingSpence's suggestion, just as one of many possibilities.
Light aircraft operating from Saints generally follow prescribed departure and recovery routes through the Cardiff Zone. Once time is allowed for this there is relatively little left for the 'meat' of the trip, so the aircraft would likely have been operating in relative proximity to each other, but still safely - deconfliction in such circumstances is hardly complicated. They would also retain a service from Cardiff ATC, who historically at least were very good at warnings of "company traffic", even when only giving a FIS. However, once their time was up, both aircraft would have likely been descending to 1000ft or so on the Cardiff QNH, and both aiming for roughly the same point geographically. There is obviously a potential conflict there.
There is absolutely no evidence to support the above theory, there are many many other possible explanations, so can we please leave it to those whose job it is to determine the cause to do so!
Pace - I'd hope you'd know from your wealth of flying knowledge and personal experience of aviation fatalities that saying that recently deceased aircrew "messed up" is offensive. Yes, it may have potentially been down to a human failing (we all know how ineffective the human eye is at spotting conflicting aircraft on a constant bearing for starters), but that is a very different beast to "messing up". Just look at the fallout from the Chinook Mull of Kintyre crash for evidence of how allegations of negligence can further the hurt of already grieving family members.