This application relates to copolymer compositions and copolymerization processes. The processes may use two different metallocene catalysts: one capable of producing high molecular weight copolymers; and one suitable for producing lower molecular weight copolymers having at least a portion of vinyl terminations, and the copolymer compositions produced thereby. Copolymer compositions may comprise (1) a first ethylene copolymer fraction having high molecular weight, exhibiting branching topology, and having relatively lower ethylene content (based on the weight of the first ethylene copolymer fraction); and (2) a second ethylene copolymer fraction having low molecular weight, exhibiting linear rheology, and having relatively higher ethylene content (based on the weight of the second ethylene copolymer fraction). It is believed that the unique combination of these properties provides a copolymer composition with advantageous viscosity modifying properties, such as an excellent combination of shear thinning and fuel economy, as well as high thickening efficiency.