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Amorphous oxide semiconductor thin-film transistors on flexible plastic substrates typically suffer from performance and stability issues related to the maximum processing temperature limitation of the polymer. A novel device architecture based on a dual active layer enables significant improvements in both performance and stability. Device fabrication occurs below 200°C on a polyethylene naphthalate (PEN) substrate using mixed metal oxides of either zinc indium oxide (ZIO) or indium gallium zinc oxide (IGZO) as the active semiconductor. The dual-active-layer architecture allows for adjustment in the saturation mobility and threshold voltage stability without the requirement of high-temperature annealing, which is not compatible with flexible plastic substrates. The device performance and stability is strongly dependent on the composition of the mixed metal oxide; this dependence provides a simple route to independently adjust the threshold voltage stability and drive performance. By switching from a single to a dual active layer, the saturation mobility increases from 1.2 to 18.0 cm2/V·s , whereas the rate of the threshold voltage shift decreases by an order of magnitude. This approach could assist in enabling the production of devices on flexible substrates using amorphous oxide semiconductors in the near future.