Gravitational wave detectors require single-frequency laser sources with challenging requirements regarding beam quality and noise properties. We developed a reliable single-frequency fiber amplifier architecture based on standard step-index large-mode-area fibers and coherently combined two high power beams to enable further power scaling. A combined power of 391 W could be achieved with a combining efficiency of approx. 92 \%. The TEM00-mode content of the combined beam was analyzed and a higher-order-mode content of 6.8 \% was measured. This yields 365 W linearly polarized output power in the TEM00-mode that is usable for the application.