Are oxygen and carbon dioxide simultaneously present in red blood cells during gas exchange?
For diffusion to occur there must be a "concentration gradient", that is one area where the substance that is diffusing from, to where it is diffusing to. Where the substance is diffusing from, the concentration is high and where it is going to, the concentration is low. Our bodies continually use up oxygen and produces carbon dioxide. Carbon dioxide is generally carried in the blood plasma, and oxygen is generally carried by the red blood cell (erythrocytes).
Whereas, it is true that in diffusion an equilibrium is usually achieved, in this case you mention there is no equilibrium. Our physiological processes (respiration) continually maintains that concentration gradient and so carbon dioxide always flows out and oxygen always comes in. Too much carbon dioxide in the blood will raise the pH and cause serious metabolic problems with enzymes and their efficient function.