Prosecutors said Khaw Kim-sun, a Malaysian national, left the inflatable ball in the boot of a car where the gas leaked out and killed them in 2015.
Police were initially baffled when they found the pair dead inside the vehicle.
Mr Khaw denied the murders, and told police he had been planning to use the ball at home to get rid of rats.
After almost seven hours of deliberation, the jury of five men and four women reached a guilty verdict.
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