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.
Try splitting your piece up into sections. You'll find in all of your piano pieces, even in contrapuntal works like Bach inventions, that there are natural breaking points where things change, whether rhythmically or harmonically or in the overall feel of the section. Try to find several per piece, depending on the length of the piece.
Then learn your music a section at a time, rather than just front to back, and vary your practices by changing the order of your sections each practice. For instance, practice section one, then section three, or start at the last section and work to the front.
This way, if you lose your place, rather than starting all the way at the beginning, you can reset to the beginning of that section. In that way, each section break becomes a milestone of sorts.
Hope this helps!
