Metallocene and other single-site catalysts can be combined to produce polyolefins with broadened distributions of molecular weight, chemical composition, and long-chain branching. These resins are finding increasing applications because of their enhanced properties compared to ones made with conventional Ziegler-Natta catalysts. Resins with bimodal molecular weight distributions (MWDs) have especially attractive mechanical and rheological properties. Although the use of these resins is expected to increase, there are very few studies available to quantify MWD bimodality or to decide a priori which combinations of metallocene catalysts will lead to the formation of polyolefins with bimodal MWDs. In this article, a necessary condition for the production of polymer with bimodal MWD using two single-site-type catalysts is derived. Additionally, a bimodality index is defined to quantify MWD bimodality.