Having 2 types of mitochondria
As far as it's accepted nowadays, our mitochondria, the organelles that allow us to breath and the ones which require the oxygen, were once bacteria that engaged in aerobic respiration, which were then fagocited by an ancestor euchariotic cell, developing a relationship of symbiosis with it and essentially turning into a cell organ. The reason plants can engage in photosynthesis is because, as we suspect, they also ate another bacteria which was capable of photosynthesis.
Now first, why O2? Well, oxygen is used as the final electron acceptor in our respiration cycle, being combined with hydrogen to form water and provide the energy to form ATP, so to breathe CO2 we'd also need to be able to use it as an electron acceptor. Luckily, CO2 already has that use in some bacteria which do not use oxygen. These bacteria, being known as methanogenic, combine carbon dioxide with hydrogen to form methane and water, and usually thrive in swamps and other locations in which oxygen isn't as widely available.
So one possible scenario to enable us to use both particles would require a change all the way back to when we were unicelular organisms, consuming a variation of our methanogenic bacteria, as well as the one which would become our mitochondria, could potentially allow for an entire planet of creatures which can use both oxygen and carbon dioxide for respiration. This use of CO2, however, requires more hydrogen than the use of oxygen and releases methane in addition to water, methane being considered an asphyxiating gas in high concentrations, meaning your new humans would need special metabolic processes to deal with this chemical, so I'd assume it's use to be facultative and limited to low oxygen environments in which CO2 and hydrogen rich food are abundant.
Note: although it sounds like a small change, the ability to use CO2 (a gas that's toxic in high enough quantities and is commonly responsible for decreases in blood ph in our species) means these humans WILL have a group of differences in their metabolism. So these humans, as well as the other animals, will not be just an exact copy of us and our animals with a second different group of mitochondria. The mere evolution of such a trait would show it was beneficial, hinting at the existence of certain environments in the world from which these humans came from that have low levels of oxygen but high enough levels of CO2 to allow the body to use it to produce ATP, or other different kinds of pressures that made it so this trait was selected as advantageous.
So summing up, your humans, like most other animals in your world, will have 2 different kinds of mitochondria, will be able to breathe both oxygen and carbon dioxide and will likely have some metabolic strategies to accommodate this ability as well as to deal with the methane, as it will be constantly flowing through their bloodstream until it's ditched out in the lungs. There's also a chance they might evolve to be capable of absorbing more hydrogen from food.