Mary Chapin Carpenter and Shawn Colvin. Shawn Colvin and Mary Chapin Carpenter.

I expected that one would perform as the warmup act and the other (presumably MCC) would be the headliner. But Wednesday night at Evanston's Cahn Auditorium, they took the stage together for a performance lasting over two hours.

Each lady armed herself with a guitar and a voice that remains strong in their sixth decade.

Sometimes, Mary stood in front and sang while Shawn took a seat. Sometimes, Shawn stepped forward while Mary took a break. But they were at their best when they performed together - alternating verses and harmonizing as if they had been singing together for years.

Of course, the old friends have been singing together for decades. It showed in their music, and it showed in their banter. They debated the merits of having a tuning device. They joked about their choice of clothing. And they finished each other's sentences, as old friends do.

Both women are accomplished songwriters, but this evening included their interpretations of many other songwriters. They played the music of folk singers like Steve Earle ("Someday") and Lucinda Williams ("Passionate Kisses"). And they played some surprises, including The Backstreet Boys' "I Want It That Way" and The Beatles' "I'll Be Back". Each song was delivered with the heartfelt passion of two performers who continue to love their craft.

And their music.