Stable low-noise high-power lasers are indispensable in advancing the strain sensitivity of interferometric gravitational wave detectors. Advanced LIGO and Advanced Virgo are currently under commissioning and require about 200 W of single-frequency laser power, while the future detector design may require up to the order of 500 W. In this Letter, we present the design and, to the best of our knowledge, the first experimental demonstration of the laser system for Advanced Virgo that is based on coherently combined fiber laser amplifiers. We show the long-term performance of two 40 W fiber laser amplifiers, as well as their characterization in terms of beam quality, power noise, phase noise, and beam pointing. Moreover, a simple and compact setup utilizing fibered modulators and actuators for the coherent beam combination of these two fiber laser amplifiers is reported. A combination efficiency of about 96% was achieved, and no spurious noise was observed.