The short answer is: I don’t think Paul was thinking about business partnerships, but that doesn’t mean we shouldn’t. Paul didn’t live in an entrepreneurial, capitalist society, and the distinction we have between being a business partner, an employee, a supplier, a customer (or even a slave!) simply didn’t exist in the ancient world. His main point of application was the withdrawal from idolatrous practices (6:14-16), as we also see at length in 1 Corinthians 8-10, but we know he was also concerned with marriage (1 Cor 7), the infiltration of sexual and financial norms into the church (1 Cor 5-6), and various other things that he writes about in both letters. If we want a real world case study of how the “yoked with unbelievers” principle works in a tangible situation, the best one might be the example of buying food in the meat market and eating in a private home (1 Cor 10:23-11:1): feel free to participate as long as nobody’s conscience is violated, it doesn’t lead anyone into idolatry or sin, and it can be done to the glory of God.

Applying this text to business partnerships is more a question of wisdom than law, I think. If my Dad had started his business with two unbelievers instead of two Christians, it probably would have made the Christian ethics of his company far harder to instil, and it would almost certainly have been an unwise decision. Investing in a large multinational company alongside many non-Christian owners—which technically is the same thing, on a far smaller scale—is something we all do, directly or indirectly, and I think that’s fine. Exactly where the line is takes wisdom, and should be triple-filtered through Paul’s principles: conscience, idolatry and the glory of God. That’s a starter for ten, anyway.