UK hours of rest require that everybody on Board ship should have a minimum of 6 hours continuous rest, not even 1 minute less, in every 24 hour period, all that time has to be away from the workplace, i.e. Bridge or Engine Room.
They also require that nobody should remain in their place of work for a minute longer than 6 hours without a minimum of 30 minutes for a Meal Break away from the Bridge/ Engine Room.
To comply with this, Watch keepers would have to pass each other in the Bridge Doorway or Engine Room entrance without stopping to speak to each other.
However, STCW requires a handover period of 15 minutes between watch keepers, during which time both are working simultaneously, if this is complied with, then watch keepers work 2 x 6 1/4 hour periods and have 2 x 5 3/4 hour rest periods.
Taken together it is impossible for any Ship to operate a 6 on 6 system off Legally.
I raised this with an MCA inspector in Aberdeen, on a British Flag ship (PSV) where everyone worked 6 on 6 off and he told me that they had had a discussion about this in the Office but none of the Surveyors could see a way to do it legally either, but all the ships in the North Sea still work 6 & 6.
As someone says on this site;- " It's all about the Money....)
It is always better to ask a stupid question than to do a stupid thing.