why does the RAF need two types?
The optimum but very expensive solution is to train helicopter 'crews' on the aircraft they will fly operationally.
'Pilots' do not require a complex aircraft to learn the skills of rotary flight and therefore the cheaper the runnng costs, the better. Some countries use two seat helicopters for this purpose, the U.K. used Gazelle and in turn Squirrel helicopters. Both did as asked of them. All operational helicopters in the UK are multi engined and therefore it is logical though not essential to train pilots from the onset on a multi engined aircraft.
The introduction of the 'crew' must provide a platform that is not so sensitive to weight distribution that prohibits pilot - crewman - crewman instructor - winchman - 'survivor' all being on the right side of the aircraft with no counterbalance on the left. Clearly not the job for a small helicopter; and few medium helicopters can cope with deliberate imbalance as previously described. The primary risk being loss of cyclic control as the stick hits the stops, whilst lives are at the end of a wire. Typically on an instructional sortie, the student and instructor crewmen are akin to me and my shadow and so the weight change across the cabin is usually the price of two people. Freedom of movement across a cabin is essential in training for both rear crew members, as students will be allowed to make errors without instructor intervention until such time as a situation develops to the point of danger. This means that on rare occasions the two rear crew members will be crossing the cabin in opposition to one another in haste.
A footnote should be that nations of the world use their helicopter crewmen / flight engineers in different ways. Some (French - Russian) rarely leave the cockpit. The U.K. (& others) has a crew member that on many occasions have actively contributed to accident avoidance and Flight Safety, and the training syllabus that has achieved this should be nurtured, not undermined by austerity.