Statistically, I would think that it "smooths" out the signal at the output.
Having two dac is equivalent to summing the output level of the DAc, average them (like (a + b)/2), then send them out to the output. It's like taking the average of two signals.
Whenever you take the average of anything, you reduce the peak and dip of that things that you try to average.
It's like the average income of people who live in California and people who live in Nevada is more or less the same. But if you look at the income of each individual in each state, you would see a lot of "peak and dip".
The same applies for electrical signal when you try to average them.
I have a Cary 303/200 and it also uses 2 dac per channel.
The Cary 306 uses up to 4 dac per channel and I think that's why it sounds smoother than the 303.