what about training the front seat observer to PPLH standard?
For precisely the same reason we don't have PPLs in the right hand seat of airliners/HEMS/SAR/Coastguard?
Have one pilot, or two. But not one and a quarter? It's not just about taking control during an incapacitation. Police flying is amongst the most challenging I have ever done, and the cockpit authority gradient needs to be fairly level, or just don't bother.
I have a funny feeling this tragedy will mark a step change in Police operations, perhaps along the lines I have just suggested?