Tomasz Procko, 22, and Karol Symanski, 29, were hauling a sofa up over a balcony when they fell to their deaths.
They died when 130-year-old railings they and three colleagues used to support the sofa gave way in November 2014, the Old Bailey heard.
Martinisation (London) Ltd denies two counts of corporate manslaughter.
The firm, which also denies health and safety breaches, has a "long and unhappy history of neglect of health and safety", the court heard.
Boss Martin Gutaj, 43, from Brentford, is accused of breaching health and safety laws, which he also denies.
The major project in Cadogan Square - located between Harrods and Sloane Square - was behind schedule and Â£400,000 over budget when the men died, the prosecutor, Adrian Darbishire QC, told jurors.
The five workers were unsupervised when they lifted the 18 stone (115kg) sofa 20ft (6m) onto the balcony using ropes, relying on Victorian railings for safety.
The two men fell to their deaths when part of the railing gave way, jurors were told.
A third was "saved only by being grabbed by his colleagues as he began to fall".
"It was an accident, but it was an entirely foreseeable and preventable one," Mr Darbishire said.
The "dangerous" sofa hauling method "was the result of a long and unhappy history of neglect of health and safety at the company which employed them," he added.
The company and Mr Gutaj had been warned "more than once" of the importance of having trained, competent and effective supervisors before the accident, the court was told.
The entire workforce spoke Polish as their first language, yet the firm's policy documents and risk assessment were only available in English, the court heard.
The trial continues.
The deaths of two workmen as they renovated a multi-million pound flat were "entirely foreseeable and preventable", a court heard.