How can CO2 exist in two different phases?
The $\ce{CO2}$ exists in two phases when you buy a partially-filled $\ce{CO2}$ cylinder because there is extra volume (which is needed to allow expansion as the container warms). Therefore, the pressure in the container is not 100 bar, but closer to 80 bar at 293K.
If the container had only enough room for the $\ce{CO2}$(l), then, indeed, there could be no gas phase. But if space is allowed above the liquid, then $\ce{CO2}$ evaporates, filling that space, and the $\ce{CO2}$(g) and $\ce{CO2}$(l) reaches equilibrium pressure.
On the molecular level, consider it an ongoing exchange between the molecules of liquid and gas. For the same reason, there is humidity in the air and oceans full of water, a 1 bar at sea level.