Now imagine if young Emmett saw both these Docs...from his POV, they would both be possible future versions of him (because, from the POV of anyone in 1931, the future isn't written yet)...he can choose to become either of them in the future, or he can choose a different path entirely...
Ok, this is convincing but here's a question; at the end of BTTF1, What happens when LP Marty disappears in 1985 to go to 1955?
In BTTF2 it is confirmed that there's only one younger Marty back in 1955. So younger
TP and LP doesn't co-exist, but what happens? I can think of 3 cases.
1-TP and LP somehow merges to form a slightly different Marty in 1955.
2-TP Marty gets erased so now its LP Marty's job to get his parents back together, with slightly different actions.
3-LP Marty is erased, he disappears in 1985 but never appears in 1955.
The 3rd one is the easiest one for continuum to handle. Because new personality means events in 1955 would be different. So it would be much easier to just get rid of the new one and keep past as it is. Also if it was TP who gets erased in 1955 when LP arrives, it would have to be the case when LP gets back to 1985, TP Marty would be erased again and LP would take his place. Since such things don't happen, we can conclude that it is always the original one who remains. Other selves just follow his steps but disappear in the end.
Also if it is not possible for them to follow the original traveller's steps, they simply disappear at the moment when they should be travelling as original did. This would explain why there isn't a new Marty in 1986 with Doc when he gets the Key. Since he had no reason, he probably got erased at the moment Marty went back to 1931 in EP1. This way past events are preserved and EP1 Marty survives. New Delorean created by this new Doc doesn't disappear according to this rule because what EP1 Marty travelled with was a duplicate, so Doc was able to come back with his Delorean.
So now back to 3 Martys, let's suppose Marty-A is our original here. B and C can't decide to come earlier in their life spans compared to A, because they have to follow A's steps. They also can't decide to come after in their life spans compared to A, because when they reach the exact moment A decided to travel, they would have been erased like new Marty in 1986 did. Finally, they can't come when they reach the exact moment A decided to travel because then, they would succesfully disappear to 1986 in their own timelines but they would never appear in 1986. Instead they would get erased like LP Marty. So it would be A who gets survived.
Hope I managed to explain clearly. What do you guys think about this?