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 */
/* write to DAC1 */
The above assumes you everything setup properly. You can do that with a single analogWrite() to channel, e.g.,
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.