Answer:
Each bike with both types of tire.
Half of the tires of each type will be in the front wheel and the other half in the rear wheel.
Step-by-step explanation:
To compare the wear of each type of tire in a matched-pair test, the individuals have to test each of the tires in the same conditions.
This will allow to compare the tires wear in equal conditions. If the tires are assigned randomly to the volunteers, there will be difference in the wear due to difference habits of the bikers.
The only way to implement a matched-pair test with bikes is to use one type of tire in the front wheel and the other type in the rear wheel. Half of the tires of each type will be in the front and half in the rear.
In this way, the distance traveled with each type of tire will be the same.