Understanding protein solubility is a key part of physical chemistry. In particular, solution conditions can have a major effect, and the effect of multiple cosolutes is little understood. It has been shown that the simultaneous addition of l-arginine hydrochloride and l-glutamic acid enhances the maximum achievable solubility of several poorly soluble proteins up to 4–8 times (Golovanov et. al, J. Am. Chem. Soc., 2004, 126, 8933–8939) and reduces the intermolecular interactions between proteins. The observed solubility enhancement is negligible for arginine and glutamic acid solutions as compared to the equimolar mixtures. In this study, we have established the molecular mechanism behind this observed synergistic effect of arginine and glutamic acid mixtures using preferential interaction theory and molecular dynamics simulations of Drosophilia Su(dx) protein (ww34). It was found that the protein solubility enhancement is related to the relative increase in the number of arginine and glutamic acid molecules around the protein in the equimolar mixtures due to additional hydrogen bonding interactions between the excipients on the surface of the protein when both excipients are present. The presence of these additional molecules around the protein leads to enhanced crowding, which suppresses the protein association. These results highlight the role of additive–additive interaction in tuning the protein–protein interactions. Furthermore, this study reports a unique behavior of additive solutions, where the presence of one additive in solution affects the concentration of another on the protein surface.