  We report the discovery of two mini-Neptunes in near 2:1 resonance orbits
($P=7.610303$ d for HIP 113103 b and $P=14.245651$ d for HIP 113103 c) around
the adolescent K-star HIP 113103 (TIC 121490076). The planet system was first
identified from the TESS mission, and was confirmed via additional photometric
and spectroscopic observations, including a $\sim$17.5 hour observation for the
transits of both planets using ESA CHEOPS. We place $\leq4.5$ min and $\leq2.5$
min limits on the absence of transit timing variations over the three year
photometric baseline, allowing further constraints on the orbital
eccentricities of the system beyond that available from the photometric transit
duration alone. With a planetary radius of
$R_{p}=1.829^{+0.096}_{-0.067}\,R_{\oplus}$, HIP 113103 b resides within the
radius gap, and this might provide invaluable information on the formation
disparities between super-Earths and mini-Neptunes. Given the larger radius
$R_{p}=2.40^{+0.10}_{-0.08}\,R_{\oplus}$ for HIP 113103 c, and close proximity
of both planets to HIP 113103, it is likely that HIP 113103 b might have lost
(or is still losing) its primordial atmosphere. We therefore present simulated
atmospheric transmission spectra of both planets using JWST, HST, and Twinkle.
It demonstrates a potential metallicity difference (due to differences in their
evolution) would be a challenge to detect if the atmospheres are in chemical
equilibrium. As one of the brightest multi sub-Neptune planet systems suitable
for atmosphere follow up, HIP 113103 b and HIP 113103 c could provide insight
on planetary evolution for the sub-Neptune K-star population.
