Horse eveners are designed to balance a load among multiple animals. With oxen, the yoke acts in the same way: an animal walking more forward of his teammate has more of the load and they generally, although not always, work to balance the load. However, if more than one yoke of oxen is hitched to the same load using one long chain, all bets are off. If the team in front is pulling more than its share of the load, the back team's yoke is pulled forward, the fit is compromised and any potential advantage goes away.
With that in mind, Tillers International designed a vertical evener for hitching multiple yokes together. Hard to explain, but easy to see in application, the video here shows the evener in use. (stop the video about 0:14 into it and advance it slowly and you'll see what I mean) The front team (Hershel and Walker) have more load to begin with and then the back team (Castor and Pollux) begins sharing the load. To see which team has more load, just look for which way the evener is "leaning."
Truth be told, this post is more to make sure that I have a reference for hitching using the vertical evener. Having hooked it in the wrong orientation once, having a handy place to see it done correctly is useful. It if helps someone else, too? Well, that's just gravy.