As long as each HDD is compatible with the DNS-320L, mixing HDD make/model should have no impact on the ability to create a RAID array. The only constraint is that it's best that both HDDs have the same capacity, which they already do in your case (1 TB).
For informational purposes: if, for example, you were to create a RAID-1 array with a 1TB HDD and 1.5TB HDD, the result would be a 1TB RAID-1, with 500MB of unallocated space in the 1.5 TB HDD. The ShareCenter will create a RAID array with a size equal to the smaller of the two HDDs.