Although the word "yokefellow" is out of use today, the meaning is easily understood. Most of us know a yoke is a device that connects two animals together to increase the power for the work that needs to be done.
Jesus said "Take my yoke upon you, and learn of me; for I am meek and lowly in heart: and ye shall find rest unto your souls. For my yoke is easy, and my burden is light" (Matthew 11:29-30). From a spiritual perspective, we labor together with the Lord Jesus. Among ourselves, we labor in the gospel. It is worth noting that God sees the marriage bond as "joined together" (same term) with a yoke (Matthew 19:6).
Interestingly, as Paul speaks highly of the women who "labored" with him, he uses two very different concepts to recognize their contribution. First, he describes them as sunathleo, or those who are "engaged in the contest" with him, like "a man also |strives| for masteries" (2 Timothy 2:5). Then Paul uses sunergos to describe those who have accomplished meaningful work alongside him. Titus is described as Paul’s "partner and fellowhelper" (2 Corinthians 8:23). These women had evidentially earned Paul’s respect for their commitment to the kingdom work.
Although the picture drawn by these synonymns rests on the work aspect, surely there is the assumption that those who are yoked together are anticipating a common goal. Jesus, with "the joy that was set before him endured the |work of the| cross" (Hebrews 12:2). And we labor in the kingdom since our "names are in the book of life." HMM III
http://www.icr.org/article/yokefellows/