Correct. You will need to use the Nuvo Configurator to configure each of the six Nuvo sources as NV-NNA sources, and then run the CasaTunes Setup to configure your CasaTunes sources to use both sound cards.
If you are using 2 x 5.1 sound cards, then CasaTunes Sources 1 thru 3 will be configured on the first card, and 4 thru 6 on the second.
If you are using 2 x 7.1 sound cards, then CasaTunes Sources 1 thru 4 will be configured on the first card, and 5 thru 6 on the second.
For more information on how to use the NV-NNA adapter please check the How To Guides