If Leah merited Yisachar on account of the episode with the Duda'im, on what grounds did she merit Zevulun?
Oznayim la'Torah (citing a Midrash): 'As a result of the Duda'im, two great tribes arose in Yisrael, Yisachar and Zevulun!' - Yisachar studied Torah, while Zevulun travelled overseas and provided for him - and Torah prospered in Yisrael!' 1
Oznayim la'Torah: For this to happen, it was essential for there to be a close bond of love and brotherliness between the two brothers, something which required them to be born from the same mother - See Oznayim la'Torah who elaborates.