  In an earlier campaign to characterize the mass of the transiting temperate
super-Earth K2-18b with HARPS, a second, non-transiting planet was posited to
exist in the system at $\sim 9$ days. Further radial velocity follow-up with
the CARMENES spectrograph visible channel revealed a much weaker signal at 9
days which also appeared to vary chromatically and temporally leading to the
conclusion that the origin of the 9 day signal was more likely to be related to
stellar activity than to being planetary. Here we conduct a detailed
re-analysis of all available RV time-series, including a set of 31 previously
unpublished HARPS measurements, to investigate the effects of time-sampling and
of simultaneous modelling of planetary + activity signals on the existence and
origin of the curious 9 day signal. We conclude that the 9 day signal is real
and was initially seen to be suppressed in the CARMENES data due to a small
number of anomalous measurements, although the exact cause of these anomalies
remains unknown. Investigation of the signal's evolution in time, with
wavelength, and detailed model comparison reveals that the 9 day signal is most
likely planetary in nature. By this analysis, we reconcile the conflicting
HARPS and CARMENES results and measure precise and self-consistent planet
masses of $m_{p,b} = 8.63 \pm 1.35$ and $m_{p,c}\sin{i_c}=5.62 \pm 0.84$
M$_{\oplus}$. This work, along with the previously published RV papers on the
K2-18 planetary system, highlight the importance of understanding one's
time-sampling and of simultaneous planet + stochastic activity modelling,
particularly when searching for sub-Neptune-sized planets with radial
velocities.
