What are the two sources for atmospheric CO2?
Only two?? There are quite a few. Two of the largest ones are volcanic eruptions and decaying vegetation. The main anthropomorphic one is burning hydrocarbons.
The atmospheric concentrations of carbon dioxide have changed over time, and drastically over geologic time! Equilibrium (changes resulting in a perceived steady state) shifts between different stable configurations. Venus and Mars as well as Earth have a particular state of equilibrium with respect to CO2 in their atmospheres.