As mcnels1 pointed out, it seems hazardous to connect two 1TWDCG, which possibly require more than double the power that the board is able to deliver. The maximum power draw of a 1TWDCG is, according to the specifications (last page), 1.671A at 12VDC (which translates to ~20W). The D945GSEJT specifications however state (page 19, last paragraph) that the on-board power connector delivers a maximum of 1.5A at 12V (18W).
However, I have so far experienced no problems with the setup, that could be referred to the power supply being inadequate to power the hard drives. I performed numerous reboots (which is when the hard drives would be expected to draw the most power), due to the complications with the setup concerning the Ethernet controller, without any failures. Also, massive read/write operations have been performed without any problems. For example, due to a defective sector on one of the hard drives, the drive was taken offline from the RAID1, after which I tested every sector on the drive, and then resynced the entire 1TB of the encrypted LVM volume. Also, the drives undergo weekly SMART testing, lasting for almost 4 hours, which has so far always succeeded.
This apparent contradiction between theory and practice might be due to the power consumption of the 1TWDCG being far less than the stated maximal 20W. In a product review that I found yesterday, the reviewer has a D945GSEJT system with an IDE flash drive and one 1TWGCG, and states (translated with some modifications for better conformity with English):
... I have an AC-DC adapter from Seasonic which is supposed to have 80% efficiency and, measured before the AC-DC adapter, the system draws about 20W with only the mother board and the flash drive connected. With the 1TB WD Green added, the computer idles at 23W, which rises to 25-28W when it reads/writes.This would indicate that the maximum power consumption of the 1TWDCG is really 80% of 8W, which is 6.4W. If this is indeed the case, the on-board power connector should have no problems supplying the hard drives.
Nevertheless, a word of caution is in order. As a late reply to one of my readers who considered connecting four drives to his D945GSEJT, I would advise against using the on-board power connector, taking the above information into account.