Cereal-legume intercrops (ICs) are a promising way to combine high productivity and ecological benefits in temperate agroecosystems. This study aims to apply a life cycle assessment (LCA) to ICs by testing several methods of co-product handling to ascertain the environmental impacts of the co-products. Several classic methods of allocation and system expansion were compared, and additional methods were proposed to better analyse the impacts of ICs and sole crops (SCs), taking into account complementarities between species that may provide benefits for each species as well as global benefits compared to the SCs of an equivalent area or production. Unfertilised ICs produce wheat and pea crops that have a lower impact with regards to climate change (CC), acidification (AC), terrestrial ecotoxicity (TE) or cumulative energy demand (CED) than N-fertilised SC wheat (W100 N) and unfertilised SC pea (P100 NO), regardless of the allocation method or functional unit (kg of grains of wheat, of pea, or 1 ha). Unfertilised ICs exhibited a higher and lower eutrophication (EU) impact when compared to W100 N and P100 NO, respectively. Concerning land occupation (10), the results were highly variable depending on the allocation method and either increased or decreased the impact of ICs relative to the impact of SCs. Classic allocation methods strongly affected the results, and system expansion yielded surprising results, emphasising the benefits of interspecific complementarity. We then redefined our functional unit to assess the impact of ICs apart from those of SCs (equivalence of area or production). These comparisons demonstrated that ICs always allowed for a significant decrease in environmental impacts when compared to SCs based on the equivalence of production. Based on the equivalence of crop area, similar trends were observed, except for EU and LO, where there was little difference in impact between ICs and SCs.