Fuel lines - why have 2 ?

Is it actually a return line or a vapor line? I'm guessing it's a vapor line. My 68 A-body has 1 fuel line. The tank is vented at the fuel filler tube. My 73 Challenger has two lines. One is the feed and the other is the vapor line. The vapor line runs from the vapor separator back by the tank, to the charcoal canister in the engine compartment. The tank is vented through the charcoal canister.

If you look at your second line, my guess is that it runs to a vapor separator and doesn't actually route fuel back to the tank.