High-order harmonic generation from atoms ionized by femtosecond laser pulses has been a promising approach for the development of coherent short-wavelength sources. However, the realization of a powerful harmonic X-ray source has been hampered by a phase velocity mismatch between the driving wave and its harmonics, limiting their coherent build-up to a short propagation length and thereby compromising the efficiency of a single source. Here, we report coherent superposition of laser-driven soft-X-ray (SXR) harmonics, at wavelengths of 2-5nm, generated in two successive sources by one and the same laser pulse. Observation of constructive and destructive interference suggests the feasibility of quasi-phase-matched SXR harmonic generation by a focused laser beam in a gas medium of modulated density. Our proof-of-concept study opens the prospect of enhancing the photon flux of SXR harmonic sources to levels enabling researchers to tackle a range of applications in physical as well as life sciences.