  Laser frequency combs (LFCs) are well on their way to becoming the
next-generation calibration sources for precision astronomical spectroscopy.
This development is considered key in the hunt for low-mass rocky exoplanets
around solar-type stars whose discovery with the radial-velocity method
requires cm/s Doppler precision. In order to prove such precise calibration
with an LFC, it must be compared to another calibrator of at least the same
precision. Being the best available spectrograph calibrator, this means
comparing it to a second - fully independent - LFC. This test had long been
pending, but our installation of two LFCs at the ultra-stable spectrograph
HARPS presented the so far unique opportunity for simultaneous calibrations
with two separate LFCs. Although limited in time, the test results confirm the
1 cm/s stability that has long been anticipated by the astronomical community.
