Q. Does unequally yoked in the bible pertain to only marriage? Could or does unequally yoked also pertain to a business partnership of a believer and an unbeliever?
A. While the marriage of a believer to an unbeliever is probably the most extreme example of being unequally yoked, in 2 Cor. 6:14-18 Paul gave a general admonition against it, not specific to any particular kind of relationship. He said that a believer is not to be involved in any kind of a serious relationship with an unbeliever. Being a partner in a business venture comes to mind.
A principle of architecture states that the weakness in any structure will present itself under stress. This can be applied to relationships too. The Christian worldview is different from the secular one and sooner or later will become a source of contention.