This method of combinine UGen's into two channels uses the addition operator: +

{
   [
      ( SinOsc.ar(440, 0, 0.1) + SinOsc.ar( 880, 0, 0.1 ) + SinOsc.ar( 1660, 0, 0.1 ) ),
      ( SinOsc.ar(440, 0, 0.1) + SinOsc.ar( 880, 0, 0.1 ) + SinOsc.ar( 1660, 0, 0.1 ) )
  ];
}.play;

Notice that, like with the Mix Class, independent "mul" levels are not preserved.