Controversy exists regarding double-bundle (DB) versus single-bundle (SB) posterior cruciate ligament (PCL) reconstruction, with differences in multiple variables affecting biomechanical and clinical results. Our objective was to compare immediate postimplantation biomechanics of SB versus DB reconstructions to determine the relative importance of restoring both PCL bundles versus total graft volume. Twenty knees were randomly assigned to five techniques (n = 4 knees/technique), performed by three surgeons experienced in their technique(s), three SB techniques (n = 12; all-inside arthroscopic inlay, all-inside suspensory fixation, and arthroscopic-assisted open onlay), and two DB techniques (n = 8; arthroscopic-assisted open inlay and all-inside suspensory fixation). Each knee was tested in three conditions: PCL-intact, PCL-deficient, and post-PCL reconstruction. Testing consisted of a posterior-directed force at four knee flexion angles, 10, 30, 60, and 90 degrees, to measure load to 5 mm of posterior displacement, maximum displacement (at 100 N load), and stiffness. Data for each knee were normalized, combined into two groups (SB and DB), and then compared using one-way analysis of variance. Graft volumes were calculated and analyzed to determine if differences significantly influenced the biomechanical results. Intact knees were stiffer than both groups at most angles (p < 0.02; p < 0.05). DB was stiffer than SB at all angles except 30 degrees (p < 0.05). Intact knees had less laxity than SB (p < 0.03) and DB (p < 0.05) at 60 and 90 degrees. DB had less laxity than SB at all angles except 60 degrees (p < 0.05). Intact knees required more load than SB at 30, 60, and 90 degrees (p < 0.01) and more than DB at 60 and 90 degrees (p < 0.05). DB required more load than SB at 30, 60, and 90 degrees (p < 0.01). Graft volumes did not have strong correlations (r = 0.13–0.37) to any measurements. Neither group of PCL reconstruction techniques was able to replicate native PCL biomechanics. DB reconstructions were biomechanically superior to SB reconstructions; they may be preferred for clinical use when immediate post-reconstruction graft strength and stability are critical. These results were not strongly influenced by graft size differences, further supporting the PCL codominance theory.