Background The purpose of this study was to evaluate the role of a non-locking plate applied to the anteromedial surface of the proximal humerus on loads at the implant-bone interface of non-locking and locking lateral plate fixation of proximal humeral fractures with a medial gap. Methods Twenty synthetic humeri models were used. In fifteen, the proximal portion of the humerus was osteotomized to create a two-part surgical neck fracture, with a 10-mm medial gap and a 5-mm lateral gap; five models were controls. In the osteotomized humeri, five models were stabilized with a locking lateral plate (group L), five with a locking lateral plate and an anteromedial non-locking plate (group L+T), and five with a non-locking lateral plate and a non-locking anteromedial plate (group T+T). All humeri were tested under axial loading until catastrophic failure, which was characterized as complete closure of the medial gap. Stiffness was calculated using force vs. displacement curves. The data were analyzed via descriptive and inferential studies, at a 5% significance level. Results Statistically significant differences were seen among all the constructions. The combination of a lateral locking plate with an anteromedial non-locking plate (group L+T) was the stiffest construction, while the combination of a non-locking lateral plate with a non-locking anteromedial plate (group T+T) was the least stiff, even in comparison with a single locking lateral plate (p = 0.01). When the two groups which utilized a lateral locking plate (groups L+T and L) were compared, the group with additional anteromedial support demonstrated greater stiffness (p = 0.03), and stiffness values for the control group comprised of intact humeri models were even higher (p = 0.01). Conclusion Combining a lateral locking plate with a non-locking anteromedial plate provides a stiffer construction for fixation of unstable two-part proximal humerus fractures with a medial gap. Mechanical benefits of medial support with a second non-locking antero-medial plate seems to be related with better construct stability in terms of strength and fatigue, potentially reducing the risk of varus collapse of the humerus head and fracture healing disturbances.