I honestly don't know why some have 1 and some have 2, mine has 1 outlet line.
Usually a 2nd line on the sending unit is a vent line, but with the sending unit on II's near the bottom of the tank that'd just be venting liquid gasoline.
The vent to the charcoal canister on II's is up at the top as well, and it's separate from the sending unit.
It'd be interesting to trace where the second line goes on a complete car.