Also interested to know since I am looking to do this in the future when I add an SSD as a boot drive. It should be similar to my MoCoSo case. I believe there is a simple metal bracket that just puts one hard drive on top of the other, with a bit of spacing in between. Ive seen another user with this setup, but dont see where you buy it.
You might also be able to just fabricate it yourself with two larger rectangular sheets of metal (one for each side). Or just use 4 thinner metal strips, one for each mounting hole in the hdd. Here is a quick mspaint of my idea. The dots in red are where the holes would be to secure the two hdd to the bottom plate of the case.