That definitely sounds like a partnership.  You can't just split it and do a Schedule C for each share, no.  If they are operating a business together and splitting the profit, that's a partnership, and a 1065 return is required.  The only way that might not be a partnership would be if one person owned the business and was merely paying the other person a set fee for their time, but not sharing the risk/profit with them.

It can't just be reported as short-term capital gains (which would avoid SE tax), because it's considered to be a business if it's conducted with a profit motive, and with regularity and continuity.  It sounds like that is all true in this case.