Gas-fermenting acetogens can upgrade one-carbon (C1) compounds (such as CO₂ and CO) to the two-carbon (C2) metabolite acetyl coenzyme A (CoA) and convert sugar feedstocks to acetyl-CoA with minimal CO₂ emissions. Fulfilling the biosynthetic potential of these microbes requires overcoming challenges in pathway engineering. Here we design a synthetic acetyl-CoA bi-cycle—in addition to the natural carbon-fixing pathways—for C2 metabolite synthesis. This pathway produces an acetyl-CoA by fixation of two CO₂ equivalents via three functional modules acting in sequence: carbon fixation, gluconeogenesis and non-oxidative glycolysis. The pathway was examined by in silico thermodynamic and kinetic analyses. The prototypic pathway was implemented in a syngas-fermenting organism, Clostridium ljungdahlii DSM 13528, by expressing a heterologous phosphoketolase that can work with other native enzymes in the host acetogen. The carbon conversion pathway is possible under various growth conditions and is independent of the Wood–Ljungdahl pathway for the valorization of H₂ and CO₂. This study reports the improvement of carbon conversion using a reductive acetyl-CoA bi-cycle and the potential impact of redox homoeostasis in the acetogenic host for industrial applications of gas fermentation.