Geng Jinping and Zhang Yujun were to be heard in a trial expected to last one day in the northern province of Hebei, the official Xinhua news agency said. The brothers of the two men were also appealing against jail sentences of eight years and life respectively, it said.
"The trial opened this morning," a court official who refused to be named told AFP.
The four appealed for a second hearing after a well-publicised trial in January that also saw Tian Wenhua, the former head of the dairy firm at the heart of the scandal, jailed for life.
A total of 21 people went on trial in January for their role in making or selling milk tainted with the industrial chemical melamine that last year killed at least six babies and left almost 300,000 others ill.
The scandal came to light in September, causing deep international concern and leading to recalls and bans around the world of products made in China.
The Geng brothers were found to have produced and sold poisoned food, while the Zhangs were charged with endangering public security.
Tian also appealed against the court ruling that she was guilty of producing and selling fake or substandard products, and failing to stop the sale of the products after learning they were contaminated, the official China Daily said.
Sanlu Group, formerly headed by Tian, was the first and biggest of 22 dairy producers found to have sold dairy products laced with melamine, normally used to make plastics.
The chemical was added into watered-down milk to give it the appearance of higher protein content.
The final verdict could be announced in court, Xinhua reported.