Why Doesn’t Hands-Separate Practice Automatically Combine?

Hands-separate practice is useful. It lets you hear the direction of the left-hand bass, settle the right-hand fingering, and temporarily remove the load of coordinating both parts. Yet each hand being able to play its notes does not mean the hands have learned to play together. Joining adds relationships rather than merely more notes: which left-hand action cues the right hand, which notes must land together, where one hand holds while the other moves, and when attention has to shift.

Research treats bimanual coordination as a complex motor skill, not the simple addition of two independent movements. The practical inference for piano study should remain modest. Such research cannot prescribe the practice method for every piece, but it does remind us that playing together requires a new timing map. If both hands are made fully automatic in isolation and then combined at tempo for the first time, two individually correct sequences may compete for the beat.

The useful transition is not to abandon separate practice, but to combine very short fragments as soon as each part is clear. The tempo may be slow, but each attempt should identify exactly where the hands meet and separate. If one place repeatedly fails, do not only practise each hand again; find the missing cue in their relationship. Separate practice learns the material. Together practice learns the relationship. Neither replaces the other.

https://pubmed.ncbi.nlm.nih.gov/14697399/
https://www.mtna.org/Performing_a_New_Piece.html