As you would know, on long traversing pitches, having gear placed to protect the third as well as the second can take some thought, either by the second clipping the rope for the third in as he passes or by the leader using some twin or half rope methods and clipping the pieces for both climbers as he leads.

Twin ropes have some limitations on belaying a follower on just one rope (generally only used for skiing, glacier travel, low angle terrain etc.) where the impact force of a fall would be minimal. And as mentioned, don't mix twin and double rope technique in the same pitch. So belaying second and third climbers can take some thought.