We have to assume you can boot into banks 1-8 normally, since you didn't mention them.
So here's what I find on my box... If I press both of the buttons at exactly the same time, nothing happens for me either, so that's not unusual. Try pressing the power button first followed immediately by the eject button. Hold them both for about 2 seconds. The logo on the switch should turn purple and flashbios will pop right up on the screen.
I do have to remind you that you must have flashed the bios with the correct one for your version if you have a 1.6b. Also, you MUST use the standard AV cables that came with the XBox. HD cables just display a blank or black screen.
Oh, and don't forget to take off flash protect before flashing from the backup 256k bios.