Syngas fermentation is a promising bioprocessing method that utilises autotrophic organisms to convert C1 gases, such as CO and CO2, into valuable chemicals, offering both environmental and economic benefits. Despite these advantages, the industrial application of gas fermentation remains limited owing to challenges in productivity associated with gas substrates. While previous studies have focused on optimizing reactor design, mass transfer, and growth medium as solutions to these specific challenges, the direct correlation between cell viability and productivity remains unexplored. To address this gap, this study investigates the viability of the acetogenic strain Eubacterium callanderi KIST612 and its impact on acetate production across various operational modes. Unlike conventional single-reactor systems, a dual-reactor strategy was implemented to enhance viable cell retention, leading to improved process efficiency. This approach significantly increased the total carbon conversion rate to 9.30mmolh-1 and the specific productivity of viable cells to 0.13g gcell-1h-1, ultimately achieving the highest acetate titer (34.4gL-1) with >53% cell viability. These findings represent a major advancement over previous studies, demonstrating that maintaining cell viability is critical for optimizing acetate productivity. By integrating viability control into process operations, this study presents a scalable and efficient strategy to enhance gas fermentation performance, improve substrate conversion efficiency, and expand biochemical production potential for industrial applications.