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Once speed is gained, driver can pedal like normal. And the rider on handlebars can leave their legs dangling in the air. This method is quite fun. It is easy to go fast.

Summary:
If you have a mountain bike you could try having the second person sit on the handlebars. Have the second person mount the handlebars and hold on. Then have driver use legs to gain some momentum. Note Driver can alternate from which side of neck he looks on. The faster you go the easier it is to balance. For the rider to dismount all they need to do is hop off.