This is not a common expression to my knowledge although, after Googling, I see it's a quote from the poet, Galway Kinnell, suggesting it's open to interpretation. I would suggest that the scars refers to the result of previous emotional damage that causes scars in the same way as physical wounds do, so each person coming to a relationship has their own emotional scars that can metaphorically "fall in love" with each other, as two people might.

I think Kinney was expressing himself creatively. This is NOT a saying. It could mean anything you want to to mean in an appropriate context. Very abstract. I his context I think it mean that both people got hurt many time and had many emotional scars. Both people in question are flawed and you want to find find love down to those flaws. I'm talking out of my ass here:)