  Astrocombs are ideal spectrograph calibrators whose limiting precision can be
derived using a second, independent, astrocomb system. We therefore analyse
data from two astrocombs (one 18 GHz and one 25 GHz) used simultaneously on the
HARPS spectrograph at the European Southern Observatory. The first aim of this
paper is to quantify the wavelength repeatability achieved by a particular
astrocomb. The second aim is to measure wavelength calibration consistency
between independent astrocombs, that is to place limits or measure any possible
zero-point offsets. We present three main findings, each with important
implications for exoplanet detection, varying fundamental constant and redshift
drift measurements. Firstly, wavelength calibration procedures are important:
using multiple segmented polynomials within one echelle order results in
significantly better wavelength calibration compared to using a single
higher-order polynomial. Segmented polynomials should be used in all
applications aimed at precise spectral line position measurements. Secondly, we
found that changing astrocombs causes significant zero-point offsets ($\approx
60{\rm cms}^{-1}$ in our raw data) which were removed. Thirdly, astrocombs
achieve a precision of $\lesssim 4{\rm cms}^{-1}$ in a single exposure
($\approx 10\% $ above the measured photon-limited precision) and $1 {\rm
cms}^{-1}$ when time-averaged over a few hours, confirming previous results.
Astrocombs therefore provide the technological requirements necessary for
detecting Earth-Sun analogues, measuring variations of fundamental constants
and the redshift drift.
