I would try to divide pasture times, or do separate paddocks with them if you can; it can escalate to a very dangerous point. My boy Danni was at a very dangerous point when I got him, and it's been a lot of work trying to get him past his 'barn\buddy sour' issues. He also had general relaxation issues, as well, which complicated things, but, if there is any way you can divide these two horses atleast a couple days a week, so they don't rely on one another's company all the time, that would be good.
Also make sure you work with them individually each day as well, even if it's just a bit of ground work; if they see you as an acceptable herdmate, that will help them overcome some of the buddy sour-ness, even if you can't necessarily separate them all the time.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."