An earth boring tool includes two coaxially-aligned, positive displacement motors. One motor turns a pilot bit and the other turns a reamer concentric with the pilot bit. The central bit bores the center of the hole and the reamer enlarges it. The central bit is rotated relatively faster, while rotation of the larger diameter reamer is relatively slow. The tool can thus be used to bore larger diameter holes without slowing drilling rates or adversely affecting performance of the cutter elements due to higher tangential velocities.