Since scanning probe microscopes scan the probes mechanically along the samples, accuracy of lateral scales of acquired images is mainly determined by the calibration of the movement of the end point of the probes. A dual tunneling unit scanning tunneling microscope (DTU-STM) with an xy stage for simultaneous lateral scanning of both the sample and the scale-reference crystal was developed. It enables calibration of the lateral scale of the sample image under the assumption that the lattice spacing is constant. Accuracy and problems of the proposed method were evaluated by comparing images of graphite simultaneously or consecutively acquired with the DTU-STM. For simultaneously acquired images in the 10 nm range, calibration of drift rates of the tips to the samples, and tilt of the samples to the xy plane were found to be effective in improving the accuracy of comparison measurement to 98.9 +/- 1.5%, regardless of orientation. The accuracy marked a higher value of 99.7 +/- 0.25% in the direction of the line scan, since the effect of thermal drift is less dominant in the direction. Consecutive single-line scanning of 150 nm with a rate of 380 ms/line, at 30 min intervals, gave an accuracy of over 99.98 +/- 0.036%.