You shoudl be able to use analogWrite() directly. Or if you want to use the ATMEL API, something like the following will work (this writes to both DACs):
/* write to DAC0 */
dacc_set_channel_selection(DACC_INTERFACE, 0);
dacc_write_conversion_data(DACC_INTERFACE, some_value);
/* write to DAC1 */
dacc_set_channel_selection(DACC_INTERFACE, 1);
dacc_write_conversion_data(DACC_INTERFACE, some_other_value);
The above assumes you everything setup properly. You can do that with a single analogWrite() to channel, e.g.,
analogWrite(DAC0, some_value);
analogWrite(DAC1, some_other_value);
Also, note that the above only works in so-called user-selected channel mode (not tagged or flexible mode). This is the default that analogWrite() puts the chip into.
Best,
Bruce