My experience of flying with a 'Heavy Crew', i.e. two Capt's. and one F/O was that the F/O ALWAYS sat in the RHS for take off and landing and the other Capt. only helped by augmenting the cruise sector.
At any time that you fly two captains together it is the operators, (your employers), responsibility to nominate a 'commander' and the other captain will fulfil F/O duties only.