I've actually tried this, removing two tires does not work properly in balance initially. After modifying the code to only have a 2 wheel setup, the vehicle moves properly, but its balance makes it unable to drive without tipping in a matter of seconds. Maybe add a .roll/.tilt or something in correlation with speed so that the bike can lean without tipping. Just an idea.

But in essence and it's basic idea, you can modify the physics to get 2 wheels - but then you'll just have balancing issues. Hope this helps.