As I see it, you're like the reserve crew on a long haul flight.
You only book the time that you actually spend in command.
I.e. there is no double booking of flight time - that's only available to FEs, FIs and the crew of multi-crew aircraft.
From the JARs:
Operational Safety Pilot Limitation (OSL -
Class 2 only). A safety pilot is a pilot who is
qualified to act as PIC on the class/type of
aeroplane and carried on board the aeroplane,
which is fitted with dual controls, for the purpose of
taking over control should the PIC holding this
specific medical certificate restriction become
incapacitated (see IEM FCL 3.035). An OSL can
only be issued or removed by the Authority.