Interesting, I never thought about that. Isn't that the same problem as when Marty travels from 1955 to 1985, and he sees Doc getting shot and himself driving the car at the same time back to 1955? That's two Marty's in the same timeline. Then, he discovers that Doc was wearing a bulletproof vest and sees the letter from 1955.
I don't think that's quite the same because although a journey from 1955 to 1985 technically constitutes time travel from the past into the future, that 'future' was still our Marty's past and the experiences of each Marty would still play out as they did. Meanwhile the brief overlap of the 2 Martys in the same time has no impact on anything I don't think. Earlier Marty isn't aware of later Marty so does nothing differently and then he's gone. Our Marty just resumes his existence in 1985 from an earlier point than he originally left it