Bette Midler isn’t cut out for back up singing — she can only be the main event. The diva recalled during a lunch at the Mandarin Oriental yesterday for “20 Feet From Stardom,” a doc about talented second-banana singers, that the only time she sang back-up, she ruined the gig. “Me, Bonnie Raitt and one other girl [sang] at the Bottom Line to back up Gladys [Knight],” she said. “We butchered it so bad. I said, ‘I’m never doing this again without a rehearsal.’ ” Midler also pointed out that when people sing along in the car, it’s never to the lead: “You’re singing along going ‘whoop, whoop.’ ” Darlene Love and Lisa Fischer performed at the Radius-TWC event, which drew Susan Sarandon and Julie Taymor.