Exactly right there can only be one person in command. Anyone else logs co-pilot time including the check captain (provided they are in the control seat), if they are in the jump and a fully qualified crew are operating the checkie doesn't log anything at all. However as I have understood it, safety FOs weren't allowed to log the flight time either, but they are on duty and so should be regulated by CAO48/FRMS.
With a fully augmented crew. Even the second officers log co-pilot time for the entire duration of the flight (not that it counts for much anyway). Not sure about a relief captain but I would still think he/she would log co-pilot time.
