Can you clarify if you are using Windows' software RAID (disk mirroring) for two 6TB volumes, one large 12TB volume, or a hardware RAID controller for RAID 1 or RAID 10?
In my experience (I fix RAID for a living), even if two disks are the same brand and type and come from the same batch of the same lot, they will fail at different times. Moving parts are unpredictable and getting two to fail at the same hour, even the same week, has astronomical odds.
That being said, segregating these drives is still a great idea, since they are of differing quality.
A decent drive controller will allow you to select which disks are placed in which groups. For instance, on an LSI card, you can create two groups of one HSGT and one normal WD, then use these two groups to create your RAID 10 array.
As for the software mirroring, my recommendation is to add one drive of each type to the system, then boot and create your mirror off of those two. The next two disks you add would then also be mismatched. It would then be impossible to get two of the same type of disks to be in a RAID together.