To quote from the Firearms Law Guidance to Police 2002
The term “unauthorised access” has
been held to include the constructive
possession that can occur where persons
other than the certificate holder have access
to the keys for security devices, as well as
access gained by criminal entry to the
premises etc.
This gives the impression that is is not OK to share a cabinet without having all guns on both certificates. However - it then goes on to say
"With regard to shared
security, such as a firearm and a shot gun
certificate holder sharing a cabinet, separate
security provisions to prevent unauthorised
access to firearms once within the cabinet or
primary security measure, should be made.
Devices such as lockable high tensile steel
cable are considered suitable for this purpose."
This is specifically talking about a firearm (section 1) and a shotgun (Section 2) being shared - maybe it is just an example.
Personally - I think it is a grey area and if I were sharing a cabinet I would just prefer to get all guns on all certificates - it costs nothing to do and potentially can save a lot of trouble.