(37) Hands Together
How do you know that you are ready to start HT? A good criterion is HS speed. Typically, the maximum HT speed is 50% to 90% of the slower of the RH or LH speed. As a general rule, get the HS speed well above final speed, typically 110% to 150% of final speed, relaxed, and in control.
There is a world of difference in how the brain handles tasks in one hand and tasks that require two-hand coordination, which is why you learn faster by practicing the hands one at a time. HT skills require the coordination of the two halves of the brain, and there are comparatively few nerve connections between them. This is why HT motions take longer to learn. Bad HT habits are the worst because, once formed, they take forever to unlearn because they took so long to learn.
Most HT practice methods are the same as for HS, such as segmental practice, most difficult sections first, continuity rule, etc., including practicing without the pedal, softly, and staccato practice, see (50) Summary of Method, item 15. This is why the instructions for HT practice appear to be so short: they have already been covered.
One method of HT practice is "adding notes": take a short segment and play the more difficult hand HS, repeating the section continuously (cycling); now start adding the easier hand, note by note. Make sure that, as you add notes, you keep the same fingering as during HS practice.
Another method is outlining [(38) Outlining, Beethoven's Sonata #1, Op. 2-1]. It is often difficult to play HT when the rhythms of the two hands are different, such as polyrhythms [(55) Chopin's Fantaisie Impromptu, Op. 66, Polyrhythms], or if important rhythmic notes are missing [(26) Speed, Rhythm, Dynamics]; detailed instructions are given in the links.
Learning to coordinate the two hands will require a lot of practice. The preceding HS work makes this coordination easier to learn because we only have to concentrate on coordinating, and not coordinating and developing technique at the same time.
Students who start HT without practicing HS first can end up with undetected mistakes in counting, etc.. Interestingly, these mistakes usually make it impossible to bring the music up to speed. There is something about wrong timing that creates its own speed wall. It certainly messes up the rhythm and the music. Therefore, if you run into problems with bringing up the speed HT, check the counting, using a metronome.
The most important new element in HT practice is the conversation between the two hands because that is part of musicality. It is certainly necessary to synchronize the hands accurately, but that will happen automatically if you concentrate on the communications between the hands. Some pianists purposely delay one hand slightly in order to provide a new avenue for expressivity, but that is frowned upon by most teachers because it over-emphasizes the communications, just as too much sugar is bad for health. Even from day one of HS practice, everyone must practice musically [(42) Musicality, Touch, Tone, Color]; otherwise students will end up practicing "how not to perform" every time they practice. HT practice is no exception.
The biggest disadvantage of learning HS first is that, initially, the HT coordination tends to be weak. Therefore, practice accurate timing between the hands, using staccato practice. The ability to play HT at speed is insufficient; the two hands must be accurately coordinated. Therefore, once you complete a piece HT, play it as often as you can and make music, because playing finished pieces is the most important part of learning piano and developing technique. Having a repertoire that can be played at a moment's notice is the difference between a musician and an amateur because you can truly develop technique to the level of a musician only by playing finished pieces.