I think it's more case of offset starting formations rather than two independent start & finish lines. At some tracks, the position of the finish line doesn't give enough room for the grid to form up without curving around a corner so they shift the starting grid forward to fit everyone in. This just means at some tracks they start ahead of the line, at others they start before it.
"Unbelievable how silly this Formula 1 is these days, with this stupid overtakes."
—Sebastian Vettel, 2012 US GP