More than a performance, this show was an occasion, a chance to sit in the same room with two of America’s most renowned songwriters and listen to them pay tribute to their vaunted careers. Merle Haggard and Kris Kristofferson are both approaching mid-70, and over the course of his long career, each has accumulated a hall-of-fame catalog of classic country songs. Thursday night, backed by Haggard’s band, the Strangers, both men exchanged songs and banter for 90 minutes before a large crowd at the Music Hall. As far as performances go, this show was imperfect. But who didn’t expect that…