LONDON—As they prepared for the last few days of their second Olympics broadcast together, NBC co-anchors Al Michaels and Bob Costas were no longer able to determine which one of them is which, sources confirmed Thursday. “Hi, everyone, and welcome back to our continuing coverage of the 2012 Olympics. I’m Bob Costas,” Michaels said during the prime-time segment. “And alongside me is—well, it appears to be Bob Costas. Which makes me Al Michaels, I think.” After Costas responded with a genial “That’s right, Bob,” both men turned and looked blankly at the camera for several seconds until the video feed cut to men’s volleyball highlights.