Why doesn't the concentration of one ion affect the concentration gradient of another ion across a plasma membrane?

In the initial stages, some ion channels across the plasma membrane open to allow for ions to flow down their concentration gradient into or out of the cell. I understand that each ion’s overall electrical potential across the membrane would affect the other ions’, thus affecting their transport across the membrane. However, I don’t understand why the concentration of ions aren’t taken as an overall concentration because surely, if we are looking at diffusion of ions caused by Brownian motion, then all molecules on either side of the membrane would affect the other molecules on that respective side of the membrane?