This paper presents a series of table-top and sled tests evaluating the thoracic force-deformation response when a force-limited, second shoulder belt is added to a 3-point belt system. The ratio of forces applied by the two shoulder belts is evaluated from 0 (a single shoulder belt) to 1 (equal force applied by both shoulder belts). A Hybrid III, a THOR, and three human cadavers are tested in a table-top environment. THOR sled tests are used to evaluate the findings in a dynamic impact environment. The table-top tests show that the effective stiffness of the human thorax increases as a function of belt force ratio, with the slope of the function decreasing as belt force ratio increases. The THOR mimics this concave-down shape, but is more sensitive to the addition of a second belt than is the human. At a belt force ratio of 0.2 (i.e., when a second belt having 20% of the force limit of the first belt is added), the THOR’s effective stiffness increases by 80%, while the human’s increases by only about 20%. This indicates that THOR may tend to over-state the benefit that a human would derive from the addition of a supplemental second shoulder belt in a frontal impact. The Hybrid III response did not have the concave down shape. Instead, its response was linear and much less sensitive to the addition of a second shoulder belt. At a belt force ratio of 0.2, the Hybrid III effective stiffness was only approximately 2% greater than with a single belt. The THOR sled tests are consistent with the table-top tests in that pronounced reductions in maximum chest deflection were observed with the addition of a second shoulder belt.