Some argue that partnerships (as a legal arrangement) are a bad idea in any circumstance. But that's somewhat off topic.
For those who are against partnerships and "direct business relationships" with unbelievers under most circumstances, what about a corporation? It's easy to point out potential problems with a two man partnership, but what about some venture where there are dozens if not hundreds of people involved? What about an employee owned company or a stock company? Does it have to be a "Christian" company? Or how big does a company have to get before working with or sharing ownership with unbelievers is no longer considered an unequal yoke?
What about a family business? Must a believer leave a family business because his father is unconverted, etc?
I am an insurance agent by trade. To outsiders, the insurance business, especially as an independent agent, somewhat resembles multi-level marketing. To contract with most insurance companies, going through a marketing company is required. As far as I'm concerned, that's a "direct business relationship." In many cases, a lot is on the line in that kind of relationship. Am I in the wrong if any of my "uplines" is not a Christian? Some of them are fairly large operations but others are quite small with regard to the number of people they employ.
I've also known a good many professing Christians who are some of the biggest crooks you can imagine. (If someone who you don't know and who doesn't know you plays the "Christian card" within the first 10 minutes, best to run the other way!) On the other hand, I've known non-Christians who conduct their business in an exemplary manner.
What if a man appears to be an exemplary Christian when you go into business with him, but he later proves otherwise. Let's assume it's in a way that does not affect business operations and has little or no effect on the company's perception by others. Must the partnership be dissolved?
I think these examples prove that the question is a lot harder to answer than some may imagine.