This study assesses the policy/legal status of both multistream residues and potential secondary products (“symbiosis products”) and whether there could be environmental benefits associated with the utilization of residues from integrated pulp and paper and carbon steel mills as raw materials for such secondary products. Waste-related European Union (EU) and Finnish policy and legal instruments were reviewed to identify potential constraints for, and suggested next steps in, the development of potential process industry residue-based symbiosis products. The products were soil amendment pellets, low-grade concrete, and mine filler. A global warming potential (GWP) assessment and an exergy analysis were applied to these potential symbiosis products. Some indicative GWP calculations of greenhouse gas emissions associating similar and/or analogous products based on virgin primary raw materials, more energy-intensive processes, and the alternative treatment of these residues as wastes are also presented. This study addresses GWP, exergy, and legal aspects in a holistic manner to determine the potential environmental benefits of secondary products within the EU legal framework. The GWP assessment and exergy analysis indicate that the utilization of multistream residues causes very low environmental burdens in terms of GWP. The utilization option can have potential environmental benefits in terms of GWP through process replacement and avoided landfilling and waste treatment impacts, as well as potentially through emission reductions from product replacement if suitable and safe applications can be identified. Waste regulation does not define the legal requirements under which utilizing residues in such novel concepts as introduced in this study would be possible, nor how waste status could be removed and product-based legislation be applied to the potential products instead.