The server at BB actually told me that if you share, there is no cost because they just bring a plate and you split it yourself and don't get extra sides. If you ask for a split order, you pay a fee because they bring each person a complete order of the sides and split it up for you before bringing it to your table.
We actually share a MC between 4 of us at BB! I and each of my 3 girls get 1/4 of the sandwich, which is conveniently cut already into quarters, and share the sides. It is very rich so that is plenty for all of us. If you look at it that way, the cost is very reasonable at $6 pp, LOL!
At CO, the MC does not come with sides, so the cost is less but you also get less food.