Pound chicken breasts between sheets of wax paper to a thickness of slightly over 1/4-inch. Put 6 or 7 fresh sage leaves on the less smooth side of each pounded chicken breast. Cover leaves with prosciutto, and pound lightly to make them adhere. Refrigerate 10 minutes.
Divide oil between 2 heavy, large skillets over medium-high heat. Dredge chicken in flour and shake off excess. Saute chicken about 2 minutes per side or until color changes throughout. Remove chicken. Cover and keep warm.
Divide wine and marsala between skillets and boil, scraping up browned bits, 1 minute. Combine in 1 skillet. Add stock and boil until reduced to 1/3 cup. Add chopped sage. Simmer 1 minute. Add pepper to taste; salt may not be needed. Pour sauce over chicken and serve.