In BTTF 2, in the second timeline where Biff went back to 1955 to give the sports almanac to younger Biff, it doesn't make sense that there would be an original Marty running around.
This is my reasoning: Since in the original timeline, Doc Brown was just a local inventor in 1985, Marty was able to go back in time because Doc Brown showed him the time machine at night, which led to the terrorists killing the Doc and forcing Marty to go to 1955. In the altered timeline, however, the Doc was committed before 1985, which means that it's possible that he never managed to build a time machine at all, or even if he did, Marty wouldn't have gotten it. In this case, alternate-Marty wouldn't have been able to go back in time in the second timeline.
This is also a fact that Doc fails to reference when he's explaining the timelines. He forgets to talk about the original timeline - The 'lame parents' timeline which started the thing. Instead, everything should be fine exactly until November 5th, where the two timelines diverged. And so from November 5th-12th, there is the 'cool parents' timeline happening where Marty is back.
But the existence of the rich-Biff timeline negates the 'cool parents' timeline from existing at all.
This would imply that when normal-Marty went back to 1955 for the second time, he wouldn't have seen his own self because alternate-Marty never time travelled at all. Instead, the events would have played out exactly as they did in 1955 before Marty ever intervened, that is, George got hit by the van and the normal timeline would have been conserved. The only difference that should have been was that Old-Biff was giving Young-Biff the almanac.
Marty shouldn't have been able to see himself. Instead, he would have seen events play out exactly as they did in the 'lame-parents' timeline - George never learnt to be brave, there would be no drama about the enchantment under the sea dance.
Is this a plot hole, or is there some in-universe/retcon explanation for this?