Richard Clarkson, 29, and Stuart Jordan, 50, who worked for Bodycote HIP Ltd at a Hereford metal refining plant, died in June 2004 after an argon leak.
A judge has now ruled the coroner's summing up had been inadequate and there would have to be a fresh inquest.
Bodycote had said it had been "shocked" at the original verdict.
A jury at a hearing in 2006 came to the verdict on the basis there had been "gross negligence" in the way the company enforced safety standards.
However, Bodycote launched the High Court challenge to look at the case again.
Suffocation risk
High Court judge Mr Justice Blake ruled the verdict could not stand because Herefordshire county coroner David Halpern gave insufficient directions to the jury over the definition of corporate manslaughter.
However, the judge said that even legal textbooks failed to provide the necessary guidance on the subject.
Bodycote, based in College Road, works in treating materials with heat and pressure.
Mr Jordan, from Bishopstone, had worked as the site's works manager and Mr Clarkson, from Madley, was a maintenance engineer.
Both were in a pit in which metal processing took place when the gas leaked in.
The two men were found lying unconscious by a colleague and were recovered by firefighters using breathing apparatus, but died later in hospital.
Argon, which is colourless, odourless and heavier than air, had been used in the refining process.
The original inquest heard how the inert gas could be deadly in confined spaces by ridding the air of oxygen and leaving people to suffocate.
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