Looking through the FAQ from last year, it looks as though you just need to meet one of the following criteria to be allowed to do both at the same time:
• Majored in math, economics, engineering, physics or chemistry
• Minimum quantitative GRE score of 750 out of 800 or quantitative GMAT score of 49 out of 60;
• "B-" or better in an intermediate-level microeconomic and/or intermediate-level macroeconomic course.
Sounds to me like they're just trying to prevent anyone without a strong quant background or prior knowledge of econ from taking both at once. Students that haven't studied econ for years are probably the only ones who'd opt to retake intermediate level courses.