No, you probably don't need to replace both tires at the same time. This is due to the fact that one's function has no bearing on the others. Because of the way the motorcycle works, the rear wheel wears down nearly twice as fast as the front wheel.   However, there are two situations in which you should replace both tires at the same time.  When your new bike's manufacturing date has passed five years, and your tires appear damaged upon inspection..  If you wish to move to a new tire brand, you may choose to change both tires at the same time.