A downhole optical sensing system can include an optical fiber positioned in the well, the optical fiber including multiple cores, and one of the cores having a Brillouin scattering coefficient which is different from another one of the cores. A method of sensing strain and temperature as distributed along an optical fiber can include measuring Brillouin scattering in a core of the optical fiber disposed in a well, and measuring Brillouin scattering in another core of the optical fiber disposed in the well, the optical fiber cores being exposed to a same strain and temperature distribution in the well.