(a) Two or more motor carriers regularly engaged in the transportation of passengers on through buses and on through tickets in pool service, at their option and with the consent of the commissioner, may make joint reports of their entire operations in this state. The road taxes imposed by this article shall be calculated on the basis of the joint reports as though the motor carriers were a single motor carrier. The motor carriers making the reports shall be jointly and severally liable for the taxes.

(b) Joint reports authorized by this Code section shall show the total number of miles traveled in this state and the total number of gallons of motor fuel purchased in this state by the reporting motor carriers. Credits or refunds to which the motor carriers are entitled shall not be allowed as credits or refunds to any other motor carrier. Motor carriers filing joint reports, however, shall permit all motor carriers engaged in this state in pool operations with them to join in filing joint reports.