The Asrock N100DC-ITX uses a 19v power brick so no separate PSU to power the drives. It comes with a 4 pin cable that splits into 2 SATA power.
The case I chose, Silverstone SG13, comes with a 3.5" HDD bracket but you need to mount the drive belly up so you can screw it in. The second drive I want to put where the PCIe slot is, found a nice 3d model for that. The thing is, the power cable would be too short and the drives would be too far apart to reach both.
So I removed the HDD bracket and cut 2 L-shaped aluminum stripes and I screwed the drive belly down. This way the cable now reaches both drives. I know, I could have cut the cable and made it longer but I prefer to avoid playing with power cables if I can.
Turned out quite good. Wanted to share it with other people. Now I need to find someone to 3d print the PCIe HDD holder and find some rubber stuff to put under the L-shapes to dampen the sound of the drive. This is the 3d model, https://www.printables.com/model/385403-pci-slot-hdd-bracket but V2 with the nice feet.
PS the drives in the images are broken so I just used them as dummy drives.
That's about it.
The risk was poorly-made power adapters, where 2 pins could short/arc. These cables were almost exclusively injection molded and sold through cut-rate outlets like Ali Express (and Amazon resellers of them).
Technically crimped cables are not immune, but they are rarely seen with such cost cutting. Quality molded cables are also fine, but that is very difficult to recognize or verify after purchase.
The manual should tell you the power limits, and you can cross-reference the power usage of the drives. It's probably ok for normal consumer drives, but high-draw or enterprise drives could be an issue. You could also (probably) just get power from the PSU.
Unfortunately it doesn't say in the manual. It only says that if you use 4 HDDs you should use a 90W DC 19V power brick. I already ordered one, my confusion was if the 4 pin cable supports 2 HDDs but I guess since the board has 2 SATA slots and cable also has 2 SATA power connectors. I plan to most likely use 2x WD Red Plus 8TB, they use 6.2W (load), 4.1W (idle)
Thanks.