quote from Marik (sorry dont know how to quote between thread)
The "problem" with thirds and double notes is that you have to find a right coordination, when your hand stays completely relaxed. Very often, with someone with even loose technique, as soon as it comes to double notes, the hand becomes like a shovel.
Another problem is control over "weak" fingers. I'd start with that. First, play trills with in triplets with 3-4, and 4-5, with accents on each downbeat. Than chromatic scales with only 3-4-5. Work slowly and find the shortest way of fingers going into the keys. Don't raise your fingers even 0.5 mm--it is waste of energy, and insufficient work. Once you reach a key bottom, immediately take any weight, leaving only as much, as it enough to hold the key down. You hand should feel calm and relaxed. In fact, it would be actually a good idea to start with stronger fingers, as it will be easier to see and analize the "feel", and then try to feel the same with "weaker" ones.
Next step, exactly the same, but in thirds--start with slow triplets trills, and then go for a chromatic scale. The tempo should be only as fast, as you can feel absolute comfort and enjoy relaxed feeling of fingers going into the keys.
Before you start with Chopin op.25/6, I'd do some Czerny. Open op.299 or op.740 and see what you can find suits your level of advancement.
Also, one of my favorite excersises is playing trills in thirds, where the upper voice is group of three, and lower goup of two, i.e:
3-4-3-4-3-4-3
1------2-------1
etc.
Then reverse them, i.e:
3------4------3
1-2-1-2-1-2-1
Then do the same with 2/4-3/5 pair.
Then modify them with triplets against duplets, i.e:
3-4-3-4-3-4-3
1--2---1--2--1
etc.,etc.
It will help you to get a "feel".
Some other excecersises would be more than appropriate. Here I cannot stress enough the use of imagination--the more you invent, the better your technique is.
Re: Legato
I strongly believe that legato is not a matter of mere physical connection, but musical connection between notes, when bunch of notes is built into music phrase--when you talk, it is much easier to say a whole phrase, rather than to spell one word. To illustrate it, take for example Joseph Lhevinne playing Czerny octave etude, or Chopin op.25/10. It is absolutely astounding--his octaves sound perfectly connected. It is just amazing, how he can create this sense of absolute, unbelivable legato in octaves.
Or take any recording of Josef Hoffman's Chopin (better, earlier recordings, when he was in good shape). All the passages he plays on staccato, but still, there is this perfect sense of legato, meaning that everything melodically is connected, creating sense of amazing perle, fluidity and clarity.
check out this
https://www.pianoforum.net/smf/index.php/topic,5485.0.htmlhow to play thirds
Topic: Thirds~~!!