It held two or more equal animals in close proximity while forcing them to labor simultaneously. The idea of partnership is involved here in the united labor. The idea of covenant is involved through the use of an inescapable binding yoke. The people to whom Paul wrote were much closer to an agrarian life than most of us today and they would have understood both concepts of partnership and covenant in the yoke analogy. If the application of this analogy includes both of these concepts or either one separately then we have to consider marriage.
Marriage is the closest of all human partnerships.
It is also the most powerful and intimate of covenants among men. In the analogy, as in real life, an unequal yoke can wound or destroy both animals involved. It also hurts the efficiency of the work. In conclusion, the idea of unifying belief with unbelief in a marriage covenant is purely foolish. Both Old and New testaments teach us this.