The biotechnological production of fuel and chemicals from renewable, organic carbon-rich substrates offers a sustainable way to meet the increasing demand for energy. This study aimed to generate platform chemicals, which serve as precursors for the synthesis of fuels and various materials, along with green hydrogen (bio-H₂) by co-fermenting two different waste streams: brewery spent grains and cheese whey (CW). Reactors fermenting a fixed quantity of brewery-spent grains were loaded with CW at 20, 30, and 40 g COD per L, and microbial production of short-chain (SCCA) and medium-chain carboxylic acids (MCCA) along with bioH2 was assessed. The reactor with the highest organic load (40 g COD per L) produced the highest amount of SCCA (21.67 g L⁻¹) whereas bio-H2 was with 30 g COD per L (181.35 mL per day). In the next phase, the generated gas (H₂ + CO₂) was continuously recirculated within the reactor to enhance SCCA production by a further 19.9%. In the later stages of fermentation, MCCA production indicated the occurrence of chain elongation from the accumulated lactic acid. Consumption of H₂ and CO₂ during gas recirculation highlighted the role of bio-H₂ as an electron donor and acidogenic CO₂ as a precursor molecule in the chain elongation process. As a result, no external reducing agent was required and only limited CO₂ was released in the atmosphere, making the overall process more sustainable and cost-effective.