Actually the project had been planned as early as the fall of 1971 shortly after Gaye had released the What's Going On album. During the first recording sessions, a pregnant Ross complained to Motown chief Berry Gordy that Gaye wouldn't stop smoking marijuana in the studio. Even after Gordy tried to appeal to Gaye, Gaye stood his ground and refused to stop smoking for Ross. As a result, most of the album was recorded in separate studios in 1972-- Wikipedia

Also cop Here my dear and read the liner notes.