It looks like our favorite celebrity couple are about to reach a BIG milestone.  It has been reported that superstar DJ Calvin Harris and his pop princess girlfriend Taylor Swift are plotting a number one hit together (if they do say so themselves.) The couple made their first public appearance together at the Billboard Music Awards.  The pair have apparently been working closely with each other in studio and are hell-bent on bringing their individual music styles together in a chart-topping collaboration. A source told the Daily Star Sunday: "Taylor and Calvin have been speaking about collaborating since they started dating. "They've had a couple of sessions jamming in the studio and are working with producer Detail. "He was behind a string of Jay-Z and Beyonce tracks including Drunk In Love. Taylor is currently on her 1989 world tour.  "Taylor has already been in LA laying down vocals for one of the tracks which could be premiered at the American Music Awards later this year. "Musically, Calvin and Taylor are from different worlds - but they are determined to bring their styles together." Rita and Calvin split in June 2014.  This isn't the first time the 31-year-old Scot has collaborated with a girlfriend, with his ex Rita Ora famously crediting their I Will Never Let You Down collab with the demise of their relationship. Speaking to Marie Claire last year, the Poison singer revealed: "I thought he had my back and that he'd never steer me wrong. "But then I Will Never Let You Down came out, and everything started to go a bit weird. I don't know if it was because business was mixed with personal or what."