Is it not an option to run a fence down the middle of the yard, so they are separated? That way they still have each other's company, but can't get to each other to fight.
What are they fighting over, and can you remove the stimulant? Like, not giving them bones or toys unless supervised etc.
Or could you build a small run in the yard, and alternate who goes in there each day? It's not easy having a dog/s with issues, but it can be done. I'm not implying that you're copping out, or that you haven't thought it through, just offering a couple more suggestions to the mix