1/2 cup (1 stick) butter, divided
1 cup chopped shallots, divided
3 teaspoons grated lemon peel, divided (or fresh lemon juice)
1 1/2 pounds broccoli crowns, cut into florets
1/4 cup water
1 1/2 pounds rapini (broccoli raab), cut into 1/2-inch pieces
Melt 1/4 cup butter with 1/2 cup shallots and 1 1/2 teaspoons lemon peel in very large skillet over medium-high heat. Sauté 2 minutes. Mix in broccoli and 1/4 cup water. Sprinkle with salt. Cover; cook until broccoli is crisp-tender and water evaporates, about 4 minutes. Transfer broccoli to bowl; cover to keep warm.
Melt remaining 1/4 cup butter with remaining shallots and lemon peel in same skillet over high heat; sauté 2 minutes. Add rapini. Sprinkle with salt, cover, and cook until rapini wilts, about 2 minutes. Uncover and sauté until tender, about 1 minute longer. Mix into broccoli. Season with salt and pepper.