  The origin and evolution of CO gas in debris disks has been debated since its
initial detection. The gas could have a primordial origin, as a remnant of the
protoplanetary disk or a secondary exocometary origin. This paper investigates
the origin of gas in two debris disks, HD110058 and HD131488, using HST
observations of CI and CO, which play critical roles in the gas evolution. We
fitted several electronic transitions of CI and CO rovibronic bands to derive
column densities and temperatures for each system, revealing high CO column
densities ($\sim$3-4 orders of magnitude higher than $\beta$ Pictoris), and low
CI/CO ratios in both. Using the exogas model, we simulated the radial evolution
of the gas in the debris disk assuming a secondary gas origin. We explored a
wide range of CO exocometary release rates and $\alpha$ viscosities, which are
the key parameters of the model. Additionally, we incorporated
photodissociation due to stellar UV to the exogas model and found that it is
negligible for typical CO-rich disks and host stars, even at a few au due to
the high radial optical depths in the EUV. We find that the current
steady-state secondary release model cannot simultaneously reproduce the CO and
CI HST-derived column densities, as it predicts larger CI/CO ratios than
observed. Our direct UV measurement of low CI/CO ratios agrees with results
derived from recent ALMA findings and may point to vertical layering of CI,
additional CI removal, CO shielding processes, or different gas origin
scenarios.
