I agree that this would be a great feature. I was originally pleased to hear that there was a Cornell Notes template in the gallery, but when I got to see it, I realized that it's an approximation of the function, but not the format, of the true Cornell Notetaking Method page. (I'm a Cornell alum; I learned the method back when dinosaurs roamed the earth!) The recall column on the left really is essential.
I understand @PinkElephant's comment regarding the difficulty in programming things to allow for a vertical line, notwithstanding a two-column table, but this kind of feature would be so advantageous to add as Evernote formatting improves. One can hope!