Why Can I Play Piano Hands Together but not Separately?

“Why can I play piano hands together but not separately?” You have four factors that affect whether a piece will be easier to learn with hands together versus hands separate:

1. First, think about whether the piece has arpeggios that span across both hands (example 1 above). With broken chords you play the notes of a chord melodically, rather than all the notes at the same time. When the arpeggios span across both hands, you’ll find it easier to play with hands together.
2. Likewise, when you play RH/LH crossovers it’ll be simpler to learn them with hands combined (example 2 above).
3. If you have chord fragments when you play hands separate, the chords will sound better and make more sense when you put them together (example 3 below). (When you only have a piece of the chord in one hand, you’ve got a chord fragment.) For example, I have a student who’s been learning an arrangement of New York State of Mind, which uses complex jazz-blues chords. He struggled with playing hands separate. However, it went better when he put the hands together, because it sounded like the song, which also made it easier to play. Here’s a quick tip for this situation. It’ll be best to start with hands separate, but quickly transition to hands together.
4. Finally, does your score have the notes, intervals, and chords arranged so the hands don’t play at the same time? If so, it’ll be much simpler to play it with hands together. Since the hands never line up timewise, it’ll almost be like playing a single line (example 4 below).