The Issachar and Zevulun agreement is named after the sons of Jacob and has its origin in the Midrash. 
When Jacob blesses his sons in the portion of Vayechi, he blesses Issachar and Zevulun together, placing Zevulun before Issachar: "Zevulun shall dwell by the seashore; he shall be a haven for ships, and his border shall be at Sidon." "Issachar is a strong donkey, lying down between the sheepfolds."
In Midrash Rabbah, it is stated that although Issachar was Zevulun's older brother, Zevulun was given precedence because he helped his brother engage in Torah study. "Zevulun was involved in commerce, and Issachar was involved in Torah study. Zevulun would come and feed him; therefore, he was given precedence."
The scripture says, "It is a tree of life to those who hold fast to it." Issachar would gather, and Zevulun would bring in ships, sell, and provide all his needs. Thus, Moses says, "Rejoice, Zevulun, in your going out."