There is a difference between split and share. If you split the entree each person gets a full serving of the sides and at BB their own gumbo or salad, but only half the main dish, The split plate fee is for the additional side that is served for the second person.
If you tell the server you want to share, it is one meal divided between 2 people. Sometimes the meal will be divided in the kitchen, but other times an extra plate will be brought to the table and it is up to you to divide the meal. There is no additional charge for the shared meal because you are not getting any extra sides.