High molecular weight elastomers, such as ethylene-propylene-diene monomer (EPDM) polymers, are conventionally formulated with a post-polymerization oil extension to mitigate their high Mooney viscosity. Post-polymerization oil extension adds to processing costs and precludes use of polymerization facilities lacking oil extension capabilities. A low molecular weight polymer may be co-produced with a high molecular weight elastomer containing the same monomers, where the low molecular weight polymer may function in place of conventional oil extension. Polymerization methods may form a polyolefin blend comprising first and second polyolefins having a bimodal molecular weight distribution.