THE IDEA
Ever since I can remember, I've improvised on the cello. Over the years, chordal playing and explicit harmony gradually took on more importance and I found myself longing for a way to sustain more than two strings simultaneously.
I commissioned an radically curved bow from Roman luthier, Giorgio Corsini, so that I could access all the strings simultaneously. For several months it seemed the perfect solution, but gradually I found it too limited as one could play four adjacent strings simultaneously, but only adjacent strings. The attacks on all four strings were simultaneous, the colorings between the bridge and fingerboard homogeneous. And to access the outer strings the middle strings needed more pressure, which limited dynamic freedom of voicing.
Musically I needed the polyphonic possibility of playing any string of my choice in any combination with the others, to be able to control the timbre of each voice, and to have independent dynamics and articulations for each string. One bow wouldn't be enough.
I proceeded to experiment, and finally develop a way of using two bows in the right hand. The left hand is free to play chordally as well as melodically. Two bows enable bowing the strings in any combination with a large and independent gamut of dynamic and expressive possibilities.
I premiered works of my own at the Palais des Beaux Arts, Brussels, and at the Westdeutscher Rundfunk in Cologne. Later, I demonstrated this to Luigi Nono and he featured it in his Diario Polacco II. Soon after that, György Kurtag took interest and wrote his perfect jewel, Message to Frances-Marie. Giacinto Scelsi readapted his Sauh in a collaborative transcription. Jonathan Harvey, Guus Jansen, Lisa Bielawa, Jay Alan Yim, Richard Barrett, Vinko Globokar, Clarence Barlow, James Clarke, Geoffrey King, Martijn Padding, Horazio Radulescu, Sharon Kanach, Martijn Padding, and others have written for me using this technique. David Dramm and Rodney Sharman wrote wonderful works interweaving the spoken and singing voice.
TWO BOW FUNCTIONS
The two bows can move independently in a vertical sense: for example, the under bow can be very close to the bridge while the upper bow is sul tasto. They can glide smoothly to the ordinary parallel position and reverse the positions with the upper bow near the bridge and lower near the fingerboard.
The upper bow can move into col legno position (and back again) while the under bow continues in its ordinary position.
Both bows can play col legno by repositioning the bows on the strings.
The two bows can produce independent articulations, accents, durations, legati etc. For example the upper bow can play staccatto while the under is legato. The under bow can produce jete while the upper is legato and vice versa.
Cross rhythms (three against four, four against five etc) can be articulated between the bows, though, of course, the bows move in the same direction as the arm.
In general, horizontal bowings are suited to the two bows and in the case of a broad detache, the volume of the cello is doubled. Tremolandi acquire a rich and complex timbre. Bounced bowings (for example, double spiccatti) are more difficult to control; they can produce interesting rhythmic phasings.
The use of two bows in conjunction with stopped or open strings can produce multiple multiphonics of eight or more pitches.
Timbre The underbow tends to have a softer and sweeter sound resembling the viola da gamba. It can emphasize a "shadow tone" if desired, thus splitting the cello and useful for echos and hocketing effects. It can be used melodically, but switching between upperbow and lowerbow for crossed string melodic work is somewhat clumsy to execute and uneven timbrically. (Playing in higher positions on the D string gives a more homogeneous effect than switching from A to D strings.)
Using the upper bow on the A string alone isn't possible as the underbow scrapes against the large C curves of the cello.
взято отсюда
http://www.uitti.org/Two%20Bows.html