If you want, another approach would be to connect the selector to only the least significant two bits (pins) of both drives, and put a jumper across the most significant bit (pin) of one drive. Then you can still change the IDs if needed without opening the case - the drive with the jumper will be numbered 4 higher than the other.
/ETA/ Actually, that would work whichever pin you selected, as long as it was the same pin on each drive. The one with the jumper will always be binary 1 in that column, and the one without, binary 0.