Understanding oxidative metabolism of organophosphate pesticides (OPs) in humans is critical for toxicological risk assessment. Electrode-driven bioelectrocatalysis enables enzyme catalysis without added electron donors (e.g. NADPH), providing simplified in vitro models of xenobiotic metabolism. Here we report a tandem configuration of two sequential bioelectrodes that reproduces fenthion oxidation using immobilized human flavin-containing monooxygenase 3 (hFMO3) and cytochrome P450 2C19 (CYP2C19), two key hepatic enzymes involved in OPs detoxification. Glassy carbon electrodes were coated with dimethyldidodecylammonium bromide (DDAB) or DDAB-stabilized gold nanoparticles (AuNPs-DDAB) to form stable enzyme films and promote direct electron transfer. AuNPs-DDAB dispersions displayed bimodal hydrodynamic diameter distribution (∼20 and 160 nm) and a positive ζ-potential across pH 2-9. Cyclic voltammetry showed reversible redox couples for both enzymes, higher currents on AuNPs-DDAB, and a positive midpoint shift for CYP2C19. hFMO3 bioelectrodes catalysed fenthion S‑oxygenation, and electrochemical titration gave K = 32.8 ± 6.3 μM, comparable to solution assays. Tandem chronoamperometry enabled sequential conversion of fenthion to sulfoxide by hFMO3 and further oxidation by CYP2C19 to sulfone and oxon sulfone. Overall, this sequential bioelectrodes system provides an electrochemically tunable route to map pesticide oxidation pathways and profile human-relevant metabolites without added NADPH or cytochrome P450 reductase.