Joe and AJ met when the Jonas Brothers and Aly & AJ were both part of the Cheetah-licious Christmas Tour in December 2005. Joe was AJ's first kiss. AJ says their first date took place at the movies: "We ended up doing the theater hopping thing. We saw five movies that night!"
They had a major fight in April 2006, after Joe forgot AJ's birthday and then she wrote "Potential Breakup Song" about it. However, it seems like they resolved their issues pretty quickly. Their relationship was confirmed in September 2006, when a lot of personal pictures of them started circulating around the internet and AJ wrote on her myspace blog: "In regards to the recent photo posted of Joe and I...yes...it’s true that we have been dating for several months now...Life is good! :)"
But by the end of November 2006, Joe gave an interview that made it clear that the two of them had parted ways, and Aly & AJ's management confirmed their break-up on myspace.
Joe supposedly wrote "Move On" about AJ after their break-up. Many of Aly & AJ's songs off their 2007 album "Insomniatic" are rumored to be about Joe, but one of the more likely ones is "Closure", which directly references "Move On" in its lyrics.
The two didn’t speak for years but in 2009, AJ told a paparazzi that she was going to a Jonas concert that Joe invited her to, and in 2012, AJ posted a picture on instagram with Joe. So it seems they are friends or at least put the past behind.