A while ago I asked the CAA about such things. An instructor wanted to do a 'training flight' for licence revlidation with a pilot using a tailwheel aeroplane, but didn't have tailwheel qualifications himself...... He intended to 'take over as PIC' once they were in flight and before the landing, with the time as PIC being 1 hr....
The CAA told me that they weren't keen on 'in-flight PIC changes' for a number of reasons. They also agreed that an instructor must be qualified to act as PIC in any aircraft in which he/she conducts any training.
Best solution is for one pilot to log PIC from A to B to C and the other to log PIC from C to B to A.