  We demonstrate a simple system for dual-comb spectroscopy based on two
inherently coherent optical frequency combs generated via seeded parametric
down-conversion. The inbuilt coherence is established by making the two combs
share a common comb line. We show that the inbuilt coherence makes it possible
to use a simple numerical post-processing procedure to compensate for small
drifts of the dual-comb interferogram arrival-time and phase. This enables
long-time coherent averaging of the interferograms.
