The PFA exemption for coaching was ratified by making them CRIs (Class rating instructor). This qualification allows the giving of flight instruction to an existing licence holder for the purpose of renewing, revalidating and additional training in the appropriate class - in this case SEP landplanes.
The experience requirements are greater than for a full FI, but the training is a lot less - about 1 week full time.
Its a bit of a grey area, but I believe it would be legal for a SNY to perform take-offs and landings if the P1 was in a position to re-take control at any time. Thus the 'student' could obtain the experience, but wouldn't be able to log it.
The instructor signing for differences training only has to sign to the effect that you have reached a satisfactory standard.
Booking P1s is incorrect for this. If flying with a PPL/CPL then its SNY, unless they have a FI/CRI where it is Put.