  CMOS-compatible Kerr microcombs have drawn substantial interest as
mass-manufacturable, compact alternatives to bulk frequency combs. This could
enable deployment of many comb-reliant applications previously confined to
laboratories. Particularly enticing is the prospect of microcombs performing
optical frequency division in compact optical atomic clocks. Unfortunately, it
is difficult to meet the self-referencing requirement of microcombs in these
systems due to the $\sim$THz repetition rates typically required for
octave-spanning comb generation. Additionally, it is challenging to spectrally
engineer a microcomb system to align a comb mode with an atomic clock
transition with sufficient signal-to-noise ratio. Here, we adopt a Vernier
dual-microcomb scheme for optical frequency division of a stabilized
ultranarrow-linewidth continuous-wave laser at 871 nm to a $\sim$235 MHz output
frequency. In addition to enabling measurement of the comb repetition rates,
this scheme brings the freedom to pick comb lines from either or both of the
combs. We exploit this flexibility to shift an ultra-high-frequency ($\sim$100
GHz) carrier-envelope offset beat down to frequencies where detection is
possible and to place a comb line close to the 871 nm laser - tuned so that if
frequency-doubled it would fall close to the clock transition in
$^{171}$Yb$^+$. Moreover, we introduce a novel scheme which suppresses
frequency noise arising from interferometric phase fluctuations in our
dual-comb system and reduces the frequency instability down to our measurement
limit. Our dual-comb system can potentially combine with an integrated ion trap
toward future chip-scale optical atomic clocks.
