It's the country equivalent of the first time that Jay-Z and Nas performed "Black Republican" together. For a solid decade, Texan country greats Kacey Musgraves and Miranda Lambert were at odds with one another. Musgraves co-wrote the absolute banger " Mama's Broken Heart ," and she wanted to keep it for herself. Instead, Lambert recorded that song and had a hit with it in 2013. That doesn't sound like start of a bitter blood feud, but those two evidently did not like one another. Their personal issues only just ended , when the two of them came together to write and record the duet "Horses And Divorces," from Musgraves' triumphant new album Middle Of Nowhere . Kacey Musgraves just announced a big arena tour. Before the kickoff, though, she played a three-night stand at Gruen Hall, a historically significant 800-cap venue outside Austin. Those were the shows that she played with the teenage mariachi trio who were detained by ICE.…