Cook crabs one at a time if your pot isn't large enough for both.
Mince enough of the most tender fennel fronds to make 2 tablespoons, and mince enough of the bulb to make 1/4 cup; set both aside for the Fennel-Lemon Tartar Sauce.
Coarsely chop the remaining fronds, stalks, and bulb, and place in a large (at least 8-quart) pot.
Add lemon slices, 1/4 cup salt, and enough cold water to come within 3 inches of the top of the pot. Bring to a boil over high heat.
While water is heating, set crabs on a rimmed baking sheet or tray, and put them in the freezer. (This dulls their senses to make handling them easier. The crab should be well chilled but not frozen; don't leave them in the freezer more than 30 minutes.) When water has come to a rolling boil, grab each crab securely at the back of its shell, and gently but swiftly drop it headfirst into the boiling water.
Cover pot, and return water to a boil, reducing heat to medium if necessary to prevent water from boiling over. Cook crabs 18 to 20 minutes. Carefully drain. Clean crabs, and serve hot or chilled with Fennel-Lemon Tartar Sauce, Bloody Mary Cocktail Sauce, and Ginger Butter.