Polyethylenes with bimodal molecular weight distribution were synthesized by using dual catalyst systems. It is found that the molecular weight and its distribution is highly influenced by the molar ratio between the two catalytic centres, and that a synergistic effect exists, being the molecular weight of the products higher than that corresponding to the materials obtained from the isolated catalysts. What it is interesting is that the molecular weight distribution shape, the microstructure and the final polymer properties could be regulated by the selection of this ratio. Results from this study show that materials with a broad range of microstructures, crystallinities and mechanical properties are obtained. The correlations found show that although a double population of macromolecular species exists, both are able to co-crystallize to form a unique crystalline structure from dilute solution. Rheological testing point towards a pronounced shear thinning behaviour, very high relaxation times, and thermorheological complexity, suggesting the incorporation of long chain branching during the polymerisation process. In addition, simple additive models applied to the linear viscoelastic properties of the polymers, by using the rheological response of the pure components, are unable to explain the experimental results obtained, suggesting a tandem effect rather than a dual action between the two active centres.