I just thought of something else. Not to discourage you or anything but if you check out Undeads thread on changing the serial, mac, online key, and hd key, they have basically concluded that the hd key plays a role in getting on to xbox live. This means that if you have two eeproms both will have to have the same hd key to work with the same hd and therefore both can be banned.
On the other hand, you did mention a bios hack that would allow the hdkey to be hardcoded in to the bios so that would work but I think you'll be forced to take that route over just coding the same hd key into both eeproms.