This paper proposes a data link layer design method for dual-processor communication based on serial peripheral interface (SPI) bus. Advantages of the two-way SPI bus in this design include using less pins (only one line more than the SPI bus) and perfect data link layer protocol control. These can ensure transparent data transfer between the two processors. An automotive body bus controller with a display unit was used as an application. It uses ARM9 and S12 as dual-processors to achieve two-way communication. Implementation and application of the method are described. Experiments show that the protocol is effective in engineering practices, and provides a reference to other SPI applications.