We report the observation of four-wave mixing in a high-power dual-wavelength twin-core Yb-doped fiber laser. The four-wave mixing process stabilizes dual-wavelength operation. We obtain dual-wavelength operation at 1090 nm with a narrow linewidth (typically < 40 pm), signal-to-noise-ratio > 55 dB, total output power as high as 1.85 W, and a record narrow wavelength spacing of 0.12 nm.