USB Dual Bay Dock Won't Mount Both HDs

[weekend]

I have the exact same problem but the fix doesn't work for me. The versions of the files above have since changed so I couldn't test with those and had to take the ones currently on your server.
After searching if I could fix it myself it seems that CONFIG_SCSI_MULTI_LUN=y has to be set. (it isn't in https://github.com/xbianonpi/xbian-patches/blob/master/kernel/.config and I assume that is one of the changes you made)
I compiled the kernel following the instructions at https://github.com/xbianonpi/xbian/wiki/Compiling-the-Linux-kernel-&-Raspberry-Pi-firmware and changed that setting. After some fooling around (and messing up the filesystem on the SD in such a way that plugging it into my laptop caused it to crash O.O) I got the fresh kernel working just fine but it still wouldn't create the second device.
In the end I just did it manually which seems like a good enough work around for now although a more permanent solution would be nice.

# cat /proc/scsi/scsi
Attached devices:
Host: scsi0 Channel: 00 Id: 00 Lun: 00
Vendor: HDD Model: Rev: 1234
Type: Direct-Access ANSI SCSI revision: 05

# echo "scsi add-single-device 0 0 0 1" > /proc/scsi/scsi

# cat /proc/scsi/scsi
Attached devices:
Host: scsi0 Channel: 00 Id: 00 Lun: 00
Vendor: HDD Model: Rev: 1234
Type: Direct-Access ANSI SCSI revision: 05
Host: scsi0 Channel: 00 Id: 00 Lun: 01
Vendor: HDD Model: Rev: 1234
Type: Direct-Access ANSI SCSI revision: 05

The device is now usable and shows up in /dev

Lessons learned:
CONFIG_SCSI_MULTI_LUN=y should probably be set.
Creating a backup of your SD is a lot less painful than filling in a 50 char wifi password 6 times in a day.
Same bug exists in raspbmc and in my quest to fix it I found I like xbian more than raspbmc or openelec.

Special thanks to the xbian dev's and especially mk01, I would have never given xbian a chance if it wasn't for the existing bug report and quick reply about this issue.