Up to a day ahead and at least a couple of hours before serving, toss the chicken in a medium bowl with 2 Tbs. of the oil, the rosemary, 1 tsp. kosher salt, and 1/2 tsp. pepper.
Heat a gas grill to medium or prepare a medium-hot charcoal fire. Divide the remaining 1/2 cup oil into two small bowls (one for grilling and one for serving). Alternately thread three pieces of sausage, three pieces of chicken, and four sage leaves onto each of six 12-inch metal skewers (or wooden skewers that have been soaked in water for 1/2 hour).
Grill the skewers, covered, until one side is browned and has good grill marks, about 4 min. Brush with some of the rosemary-garlic oil, flip, and cook the other side until it, too, has good grill marks, about 4 min. Brush with more oil and flip again. Continue cooking, flipping, and brushing with oil until the sausage and chicken are both cooked through (check by slicing into a couple of the thicker pieces), about 10 min. more.
Let cool for a couple of minutes and then arrange on a platter, drizzle on the remaining oil, and set out for guests to serve themselves.