My theory is think is more preferable. The Marty that returns from 1955 to 1985 is returning to exactly how he first left parents poor and unhappy biff being a bully to his dad doc getting riddled with bullets and dying. It is only when the Marty 2 that's in the parking leaves in the DeLorean to 1955 do the changes start to happen. Marty 2 has to first leave the timeline for the changes to happen otherwise both Marty's are stuck with poor parents and a deceased doc.