With the lack of detail, I'm going to assume that you are seeking to transfer data between two DNS-320s and both HDDs are formatted as standard volumes (6TB = 3TB HDD + 3TB HDD). DNS-320 formatted HDDs are swappable between units. Remove one of the HDDs from the original DNS-320, add a new HDD in the empty slot and copy the data from HDD1 to HDD2 internally to the DNS-320 (no external data transfer required). Repeat the process for the second HDD.
If you want to cut the reduce time by 50%, move the second HDD (containing data) to the second DNS-320 and copy the second HDD at the same time that you are copying the first HDD using the other DNS-320.