I'm not saying the "spares" being in the cockpit is a necessity but if the capt can't deal with someone being in there and looking over his shoulder, how does he cope in the sim or on a line check?
Anybody in the idea, that it would be voluntary for the augmenting crew to be present in the cockpit (unless resting) might want to doublecheck the FOM before making any further statements.
With a bit of "SA" (i.e. when to be where in relation to the Engineer etc.) it can be accomplished rather easily, as suggested by Muttley C.
My thoughts go with the crew, whatever the outcome of the investigation.
Cheers
OBOGS