Before you doubt my sanity or think a newbie has taken my name, let me explain:
Suppose a newbie was interested with FMA but confused by the two Anime adaptations- Anime1 and Brotherhood. He is informed that the Anime1 was being made when only about 20% of Arakawa's 108-chapter manga had been published. Arakawa told BONES to cover the existing material (up to the brothers meeting Izumi & Al being kidnapped by Greed) and then write a different storyline and conclusion. Since she had an idea about her story's ending, BONES were told of this as well. Therefore, they wrote the new plotline and avoided anything that might spoil what happens in the manga (which would eventually be adapted in Brotherhood). If there are some parallels, Anime1 would have to give different conclusions (with exception to the foregone conclusions of the Homunculus' defeat and what happens to Al).
This means that the newbie could watch one adaptation and deduce from its events how the other adaptation covered things. And he/she would be 80% accurate.
Take the closure of Roy dealing with the murderers of Maes Hughes (Bradley and Envy). If the newbie watched Brotherhood first, he/she would conclude that Anime1Roy will not have a final battle with Envy. Vice versa, watching Anime1 first, he/she will conclude that BrotherhoodRoy will not have a final battle with Bradley. Furthermore, Roy stopping his revenge in one adaptation will mean he fulfills it in the other.
-Scar and Kimbley have a final battle in one adaptation. They won't in the other.
-Both series have a territorial human transmutation circle. One adaptation will have the transmutation be prevented. The other will have it be successful.
-Noble Edward will not kill in one adaptation, meaning that in the other...poor Ed.
-Seeing the conclusion of Ed and Winry's relationship in one adaptation, the odds of it being repeated in the other version...FAT CHANCE!