A lot of downrigger releases are meant to go inline between the cable and the ball. As long as you use that style, you don't need the second eye.
I'll sometimes use the second eye for attractants. Depending on my presentation, I'll tie mono to the ball at the eye and then my cowbells ir something similar to a swivel on the other end. When I do that, I'll get rid of the inline release and use a stacker a couple feet above to reduce the risk of tangles.
Its really the only practical use I've found for the second eye, but most people have their own preferences.