Bringing it back to the OP, this is a common criticism of BTTF that, fittingly, a lot of people get wrong.
There’s only one Marty in the movie. When Marty changed the past so his parents didn’t get together, it took “time” (somehow) for the change to propagate all the way down the timeline and erase him from existence. When he fixes things, it sets off another change propagating down the timeline. It will, eventually, catch up to Marty, rewriting his memories so that he remembers “always” having the cool family. But there’s never a “second” Marty - there’s just the one that hasn’t been acclimated to the revised timeline yet.