A new setup of dual tunneling-unit scanning tunneling microscope (DTU STM) has been developed that can realize simultaneous calibration between test sample images and standard reference scales. The crystalline lattices and periodic grating features are employed as such scales for practical length measurements, respectively, in the nanometer and the micron orders. The DTU STM consists of a reference unit and a test unit horizontally set in parallel to eliminate Abbe's errors. Their probe tips are attached to one single XY scanner on the same surface, while the reference and test sample holders are open. As the tips scan over the sample surfaces, two images with the same lateral size are simultaneously acquired. Line lengths in the test image could be measured by counting the number of crystalline lattices or grating patterns in the reference side. Two vertical impact drive mechanisms were applied to coarsely positioned samples in the Z direction. Another two impact drive mechanisms were employed to the horizontal drive sample holders as to automatically position the samples in the lateral plane. We present a brief discussion about the concept and setup of the DTU STM. Some comparison results using crystalline lattices and grating patterns, respectively, as reference scales for length calibration are provided. Experiments show a satisfactory matching between the two tunneling units when covering a wide scan range from 5 nm to 10 mum. With its new setup, the DTU STM is confirmed to be a more accurate device for practical length measurement even when measuring comparatively large and heavy samples.