Maybe one line is the brake, and the other is the throttle? No wait, you've got the throttle up at the top there... now I see it. Maybe one is a 'clutch' to disconnect the drive and freewheel it? Hmmm.... Would be a fun ride to tool around on though.
This thread reminds me of the days of my youth when my friends and I would tie a GT SnowRacer to the back of a car by a long rope, and see how fast we could ride that racer down the snow covered streets. We dubbed it 'Canadian Surfing' and I was the stuff of legend for all of a week as being the guy with enough balls to do more than half-the-ton (80+ kph) on that sucker, going sideways, laying down sparks every time I hit a patch of pavement. We ground the runners on that racer down to mere stubs.
Man, those were the days. Good times.