a) The mounting distance between the two sata connectors is much too small for two 3.5" disks. There would have been plenty of space on the board!
b) The manufacturer wasted money on an unnecessary switch which disables both disks. This prevents booting from these disks because no way to manipulate that switch has been published. (Individually powering down each disk would be possible by hdparm commands and needs no special switch).
c) There seem not to exist any publication of the schematics of that board. This makes it impossible to understand it’s workings.
d) The documentation of v. 1.2 is lousy because working of the 12V supply is not well enough explained.
e) The ATX power pins seem of no use because not all 4 pins are appropriately connected.
I hope that Seedstudio takes notice of this unfortunate situation and promptly delivers the missing information.