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I don't know about you guys but I can only think of one thing at a time. okay say I memorize bach 2 part inventions. it's easy right? I can memorize one hand until I can sing the the line with the scale degree in mind then do the same for the other hand but the problem is I cannot synchronize the notes together.
sure I can imagine or sing one line but I cannot imagine the other line so basically I cannot just sing an excerpt of the song and be able to know what to the other hand plays.