Is it true you can’t ride double on a bike until 500km?

Is it true you can’t ride double on a bike until 500km?

It’s a fact or myth. similarly after half engine work again same has to follow for 500km , does it work for engine long life or just a myth. It is advised to ride the new bike at economical speeds before the first service as the engine takes some time getting accustomed to the fuel lines.