In one respect, Sunday’s early show at the Bluebird Cafe was par for the course. A pair of country singers, songwriters and friends picked up guitars and shared the tiny stage to trade off on tunes they had written.

There was one huge difference, of course: Those songs included “Crazy,” “Sunday Mornin’ Comin’ Down” and “Funny How Time Slips Away,” because the performers were Willie Nelson and Kris Kristofferson.