I recently began having a problem when I added an audio CD to iTunes (downloaded from the CD drive). The CD was of an album by one artist, but a second artist joined him on track 1. iTunes downloaded this as two separate albums; one with tracks 2- 10 and the other with track 1. This hadn't happened before. I can't find a control concerning this. Thanks.
Try opening iTunes, navigating to the single song that is listed under the second artist and clik on it. Go to File>Get Info, on the Info tab change the Album Artist field to the other artist and click OK. See if that doesn't organize it with the other songs.