If you want to increase the number of reds you catch remove the top shell as V-bottom mentioned. Reds don't like shells or fins.
Our experience is that removing the top shell does not catch more reds, but gets the bait picked apart much more quickly by sea catfish.
We prefer cutting whole market crabs in half both for faster dissemination of scent and to ensure better hook ups. Half crabs also tend to catch more gafftops, slot reds, and slot drum.
Some times whole crabs are better:
1. Crabs are on the small side.
2. Sea catfish (hardheads) are picking cracked crabs apart so quickly that frequent rebaiting is needed.
3. You prefer not to catch gafftops.
4. Crabs are in short supply. 1 whole crab will last much longer than two halves - usually lasting until you catch a fish big enough to eat it.