i agree.I don't understand what you're saying. Whatever age Barry #2 is, he's different from 18y-o Barry #1 and had an altogether different life. Barry would know he is in an alternate reality just from that fact. There was no reason at all for him to try and get Barry #2 powers if my understanding of the movie is correct
when he saw pics of Barry 2's happy family and not having any memories of that timeline that should instantly clue in that he's in a different reality.