Purpose: Volumetric coverage of current CBCT system is limited by machine mechanical configurations and detector panel size. To address this problem, we developed a novel CBCT image acquisition method, DSCS (Dual Scan with Complementary Shifts) by using a pair of complementary circular CBCT scans with lateral shifts and opposite longitudinal shifts of the detector panel, combined with complementary longitudinal shifts of the couch table, to provide up to 40 cm longitudinal coverage and a 45 cm field‐of‐view (FOV). Methods: The two source positions are up to 40 cm apart in the longitudinal direction. In the 1st scan, the detector panel is shifted inferiorly and its superior edge is aligned with the 1st source position. In the 2nd scan, the panel is shifted superiorly and its inferior edge is aligned with the 2nd source position. The detector is shifted laterally by 16 cm to obtain a 45 cm FOV. Opposite lateral detector displacements are used to improve image quality by providing complementary information. To support DSCS, we have developed a new filtered‐back‐projection (FBP) CBCT image reconstruction algorithm to create images from the overlapped and shifted projection data of the dual scans together. Results: We have successfully implemented the proposed DSCS CBCT acquisition method on a Varian TrueBeam radiation therapy treatment machine in the research mode. Satisfactory results were obtained for a human body phantom. Conclusion: The complementary pair scans provide a promising solution to the longitudinal coverage and effective use of imaging dose. The efficiency of imaging dose usage is near 100% and there is no imaging dose outside the effective longitudinal coverage. This new CBCT method could be useful to facilitate clinical applications including image guidance and treatment adaptation. Supported by a research agreement with Varian Medical Systems.