I had a look at ? ??? about a Yissachar-Zevulun partnership, and he goes through at length (??”? ?”? ??’ ?”?) all the various dinim, and whether it needs to be exactly half/half; whether it is ????, and a lits of different ?????.
He brings (a) a ????? about whether you can do it with somebody who is not ???? ???? ?????? (although he is talking about the Zevulun rather than the Yissachar), and (b) he brings that the partnership must be done BEFORE the learning; however if one has already learnt and tries to sell it, not only does it not work, but he loses what he already has.
http://hebrewbooks.org/pdfpager.aspx?req=922&st=&pgnum=278&hilite=