As chair of the subsidy scheme, Ms Yingluck has been charged with dereliction of duty by the NAAC in January. It charged she had ignored multiple warnings about corruption.
The two new witnesses are Somchai Sujjapongse, director of the Fiscal Policy Office, and Ampon Kittiampon, secretary to the prime minister, said Norawit Lalaeng, the PM's lawyer.
Mr Somchai will give statements against those of deputy finance permanent secretary Supa Piyajitti, on which the case was built.
He will also testify on budget allocation to the project.
On April 9, Ms Supa was chosen by a screening committee to be the new NACC commissioner, replacing a retiring member.
Mr Ampon will testify that the resolution of the cabinet had not ignored warnings or refrained from doing its duty
The two witnesses will make the investigation more complete, the lawyer said.
"To date, all new witnesses approved by the NACC are politicians. None of them are government officials related to the scheme," Mr Norawit said.
He also said his team was preparing a case to sue the NACC.
The charge would be dereliction of duty under Section 157, the same charge the prime minister is facing, as well as under Item 29 of the NACC's ethics code on the search and collection of evidence, Mr Norawit said.