Doc brown did say that the changes would ripple slowly through the timeline, so when marty 2 returns to witness himself (marty 1) travel back, he (2) would witness the gradual change (of his parents being happy) that is taking place around him. These changes would take place as marty 1 was in the past and marty 2 was moving forward from the point he returned. He would have returned to the same timeline he left but as soon as he returned and saw marty 1 leave then the changes would start to take place.