One of the technical discoveries of romantic pianism is the so-called "Cantabile with both hands". This means that the main melodic line, being played in the central part of the keyboard, is obtained by alternating the two hands, and often by alternating the thumbs.
A typical example of this procedure, which will be intensely exploited, is Liszt's famous Liebestraum N. 3.
The difficulty of correctly performing this type of cantabile is that the melodic line, being divided between the two hands, must not be broken. The meaning of the musical phrase must remain uniform, even if we alternate two different hands.
In this sense, the legato clearly cannot be performed by holding the notes with the hand, since we are not required to keep our hands adhering to the keyboard, but only thanks to the use of the damper pedal.
We must be able to maintain the dynamic arc of the phrase, and its linearity in compliance with the legato signs indicated in the score.
It is therefore necessary to avoid the production of accents, often random, not functional to the harmonic and melodic tension of the phrase. Conversely, a true legato has very few accents, and a very careful, balanced and proportionate sound management must be constantly sought. At the same time the accentuations of the phrase must sound spontaneous, diversified, and not artificial.
The phrase lean on an accent indicated by the author with an articulation mark (dash / accent), or a slur, or where the harmonic or melodic tension is greater. At the beginning of Liszt's Liebestraum N. 3, the most tense and most intense point of the phrase is certainly the chord at bar N. 4. It is important to have a unified conception of the harmony of this opening phrase; afterwards the dynamics and accentuations can be managed in a consequential manner.
The attack should not be too accentuated, in order to have a margin of growth of the level of intensity. There will be a gradual crescendo, without sudden dynamic jumps, and then a gradual diminuendo leading to the end of the phrase. Particular care should be taken not to give accents on the downbeats, that the alternation of the hands could lead easily to make.
For example when the left hand enters, the same kind of sound has to be sought, avoiding giving too direct a musical support that would create a jolt towards the top of the phrase.
Of course, to get a good legato it is very important not to play everything loud but, on the contrary, to have margins to be able to rise and fall dynamically within the phrase.
At the same time, in order to have more expressive margins, it is essential to create a clear sound distance between the melody and the accompaniment.
In the opening phrase of this song the sounds that make up the accompanying arpeggio should be kept as soft as possible. These soft tones should be melted, not too detailed. To do this, the movement of the hand, not that of the fingers, has to be used.
The more we manage to keep this voice low, the better we will bring out the singing tones. If the accompaniment is too loud, accents and forcing in the singing tones will be involuntary created. By creating a soft sound "bed", the melody will be able to soar in its natural expressive form, without any kind of forcing.
An interesting psychoacoustic exercise to try to keep the second voice in the correct dynamic range is to listen carefully to the resonances of the cantabile notes and insert the accompaniment notes within this resonance.
For example, at the opening of this unforgettable melody, the note C must continue to resonate in our ears, while the other notes are played. In this way the resonance of the note C will never be obscured, hidden by the other notes.
Dynamic control is often more about listening than about mechanical control.
Mechanical control will be a consequence of our musical intention, of what we hear, of what we imagine at sound and auditory level.
Franz Liszt: Liebesträume S.541. First edition, Fr. Kistner, Leipzig, 1850.