Seems Mary-Kate Olsen is juggling two guys. Olsen, 20, has been seeing Matt Creed, a “quiet, shy” NYC-based DJ, since April. “They’ve gone to a couple concerts,” says a source. But on June 10, Olsen was seen dining with her ex, David Katzenberg (they dated from 2003 to 2004), at West Hollywood’s Chateau Marmont. The true test: Which guy did she take to her 20th birthday bash — a joint celebration with sister Ashley — on June 13? .......David Katzenberg!