You know you have it when you look for a reason why these twins aren't twins [where is it]

Don't forget the surface pressure as well. 0.06 atm versus 0.17 atm. When it comes to Elite rendering atmospheres, I think the difference here lies in that, and not in the difference in carbon dioxide.
That one on the right looks like it might be breathable. Not pleasant, but enough to keep you alive. The partial pressure of the CO2 is about 0.918% of an Earth atmosphere, which isn't lethal, but it will feel "stuffy" and make you drowsy. The O2's partial pressure is only about 16% of an Earth atmosphere, which is breathable, but without being conditioned for it you might feel like you have altitude sickness, or at least light-headed.
We can't ignore the changes that minor differences can make over millions of years on the evolution of the life forms of the respective bodies and how they affect the atmosphere.