I've had this problem too. My friend and I spent over three hours doing this, with about 30 30 second-long races each, and neither of us got the achievement.
We'll try again tomorrow, but if anyone has a solution it would be great.
In addition, for no apparent reason, I received the achievement for winning 10 races on one's own custom track while attempting the above - yet I have not even won on my tracks at all. Has anyone else had this happen?