Monk is right:
You have to find ways to simplify one hand.
Once a student has thoroughly learned a passage with separate hands, /s/he mastered the “technique” to play that passage. Joining hands will bring up a completely different set of problems: co-ordination. Hands together is actually 37 times more difficult than hands separate.
This is one of the reason why going to hands together straightaway does not work: you have to solve the technical and the co-ordination problems at the same time.
This is the same problem as rubbing your stomach in circles and patting your head. Or having one arm making circles forward, and the other backwards. One of the main reasons why this is so difficult to start with, is that one hand tries to move sympathetically with the other. In other words: one hand interferes with the movement of the other.
The solution lies at the nervous level. You must inhibit the nerve message for the sympathetice movement, then you must have both hands move independently.
How do you do that?
Here is the single most powerful routine to do that. It targets specifically co-ordination, so you must make sure that the technique required for the passage had been thoroughly mastered by previous work on hands separate.
I call it “dropping notes”.
The student should start by playing the right hand (for example – it doesn’t really matter which hand you start). He plays the right hand only a couple of times. If it is fluent and smooth, the student is now going to add the first, and only the first note of the left hand. In other words, s/he is going to “drop” the first LH note. The aim is to keep the RH going no matter what. It does not matter if the LH note is wrong: it can be fixed later. The only thing that matters is that the RH should continue undisturbed, no matter what the RH is doing. It is going to take 2 or 3 (maybe more) repeats for the student to get the hang of it. Once s/he can do it, add the next note. To start with everything will fall apart. Never mind, keep trying. First aim to keep the RH going no matter what. Once this is accomplished, to get the LH notes right. It is all a matter of priorities. At this stage the priority is not to get the LH notes perfect at the expense of the RH movement. All we are interested in, is to get the RH to move independently of what the LH is doing. Once we accomplish this, them we can turn our attention to accuracy.
Keep adding the LH notes one by one. Every time you add a new note, everything falls apart, but as you proceed, it gets easier and easier. Eventually you will be playing hands together!
Now comes a very important step that must not be skipped. You must do it all over again, but this time the reverse: start by playing the whole of the LH, and drop the RH notes one by one. Once you can do both ways, you will have mastered HT for life!
Do not try this with the whole piece. Work in small passages, maybe one or two bars at a time. Bach fugues tend to be a nightmare to join hands. Try this method: it always work.
And here is another thing that should be done as well. Many times the student gets overwhelmed by the physical task and cannot deal with the
sound of it all. So Once they can do HS well, let them play one hand, while you play the other. This way they can “hear” how it all sounds together, without the complication of actually having to play both hands. If the sound of hands together is firmly on their minds, somehow the fingers comply to actualise that mental representation.
Best wishes,
Bernhard.