Make Rosemary-Garlic Oil by heating olive oil and garlic in a small saucepan over medium heat, stirring occasionally until garlic starts to bubble steadily, 3 to 4 minutes. Add the rosemary, remove from heat and let cool to room temperature. You can make this up to 5 days in advance. Store, covered, in refrigerator.
Up to one day ahead and at least a couple of hours before serving, toss chicken in a medium bowl with 2 tablespoons of Rosemary-Garlic Oil, 1 teaspoons fresh rosemary, 1 teaspoons kosher salt, and 1/2 teaspoons freshly ground black pepper.
Divide 1/2 cup of Rosemary-Garlic Oil into 2 small bowls (one for grilling, one fore serving). Alternately thread three pieces of sausage, three pieces of chicken, and four sage leaves onto six 12" 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 minutes. Brush with some of the rosemary-garlic oil, flip and cook the other side until it, too, has good grill marks, about 4 minutes. Brush with more oil and flip again. COntinue cooking, flipping, and brushing with oil until the sausage and chicken are both cooked through but not overdone (check by slicing into a couple of the thicker pieces), about another 10 minutes.
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.