Since the AD1853 has current outputs, these can be tied together with no
problem and the output will be the sum of the two channels. See datasheet page
16 figure 30 (mono application circuit).
The noise/distortion results depend heavily on the rest of the circuit design.
e.g. if the output op amps saturate because they are receiving 2x the current
then the result on distortion is rather terrible! The noise in the two channels
is uncorrelated so you should theoretically get an improvement in noise by a
factor of sqrt 2 (from the DAC). This will not help much if the majority of the
noise comes from the amplifiers.