This article reports results from a dual imaging-unit atomic force microscope (DIU-AFM) used for length metrology based on reference scales. The DIU-AFM, suitable for most samples with different conductivity, was developed in view of the fact that the application field of a dual tunneling-unit scanning tunneling microscope was strongly limited by sample conductivity. Two atomic force microscopy (AFM) units were combined in the DIU-AFM, one as a reference unit; and the other a test one. Their probes with Z piezoelements and tips were horizontally set in parallel at the same height to reduce errors due to geometric asymmetry. A reference sample and a test sample were attached to one single XY scanner on same surface, and were imaged by the AFM units at the same time. The two images have the same lateral size, and thus, the length of the test sample image could be accurately measured by counting the periodic features in the reference one. The most remarkable advantages of the DIU-AFM include: Scanning errors caused by the nonlinearity or hysteresis of the piezoscanner can be successfully avoided; moreover, it is able to measure conductive and/or nonconductive samples. We present a discussion about the setup and control of the DIU-AFM. Some comparison experiments, respectively, using periodic features of porous alumina film and grating as reference scales were demonstrated for simultaneous length calibration. Experiments show a satisfactory result of matching and calibration between the two imaging units when covering a wide scan range up to 8 mum. On the basis of different reference scales, the DIU-AFM is capable of realizing nanometer and/or submicron accuracy length metrology of test samples with any conductivity.