Preheat oven to 375°. Line a large roasting pan with 3/4 of rosemary sprigs. Place chickens on top.
Whisk 1/2 cup oil, lemon juice, and honey in a small bowl to blend. Using your hands and beginning at the neck end of chicken, gently lift skin away from breast. Under skin of each chicken, rub 1/4 cup lemon mixture into meat. Drizzle remaining lemon mixture all over outside of chickens.

Place 1 lemon half and 1 shallot inside each chicken cavity; stuff with remaining rosemary sprigs, dividing equally. Season chickens all over with salt and pepper.

Place remaining shallots around chickens; arrange 4 lemon rounds on top of each chicken. Drizzle shallots and lemon rounds with remaining 2 Tbsp. oil.

Roast chickens, basting frequently with pan juices, for 45 minutes. Increase oven temperature to 425° and continue roasting until an instant-read thermometer inserted into the thickest part of the thigh registers 165° and skin is deep golden and crispy, about 10 minutes longer.

Spoon pan juices over chickens, shallots, and lemon slices and serve in roasting pan.