Question

Trucks and buses have double tires on the rear end

Answer 1

The overall weight of the vehicle is distributed with some ratio between the axles. For handling and maneuverability it generally makes sense for the front steering axle to be very close to the front of the vehicle, while most of the cargo load is carried towards the back of the vehicle, both between the axles and overhanging past the rear axle. A greater fraction of the load is therefore carried on the rear axle, and in order to keep the ground pressure within reasonable limits, a larger contact patch area is required. The construction of pneumatic tires makes it difficult to make one that is very wide relative to its profile height, so putting two narrower tires next to each other is easier, and provides some redundancy in case one fails.

Answer 2

because they carry very much load than any vehicle