So is it possible to have two of the
https://www.apple.com/shop/product/...2i-8tb-internal-storage-enclosure-for-mac-pro installed?
In other words have a total of 3.5" 4 hard drives for much less than the cost of the Pegasus RAID drive?
No, only one (1) Promise 2Ji can be installed. It comes stock with an 8 TB Toshiba disk. A second disk can be installed in the 2Ji. If you add a second 8 TB disk you will have a total of internal attached 16 TB. Using Disk Utility or SoftRAID these two disks can be setup as RAID-0 if you wished.
There are brackets that can be used that act dimensionally as a 3.5 inch disk and into which two SATA III 6G 2.5 inch disks or two SATA III 65G SSDs can be installed. The brackets can then be installed into the 2Ji. Two of these brackets will fit into the 2Ji but only two 2.5 inch SATA disks or SSDs can actually be connected. Again, these two storage units can be setup as RAID-0 if you wish as stated above.
Here is an example of a 'bracket' for this purpose for having 2.5 inch storage units installed. I intend to use two of these brackets but at this time I have not received them to ensure they fit into the 2Ji. However, I strongly suspect they will, even if it means tweaking them a bit. They cost $7 each.
https://www.amazon.com/Corsair-Dual...A2E4R804PD&psc=1&refRID=17JT88BB36A2E4R804PD#
I intend to have two of the 2Ji holders each with different disks in them of different sizes. One will have two 8 TB Toshiba disks, and the other two 16 TB Seagate disks (32 TB total) for long term bulk storage requirements. My two spare Corsair brackets from Amazon will hold a total of four 1 TB OWC Mercury EXTREME Pro 6G SSDs (that currently sit idle on my book shelf). Using two of these in a RAID-0 setup will provide me with some fast internal i/o when needed beyond what I can fit into my internal Apple boot SSD.
I will format each of the big capacity 3.5 inch disks as APFS with a single Container. Within the Container I can quickly add sharable Volumes for my Projects. The APFS is fully supported on spinning disks by Apple, and the APFS Volume space sharing is a real benefit over what HFS+ provides.
I hope the above clarifies things for you.
