Thanks to threegun, luper, and others, I went to the range today and did some more double tap drills. I moved the target closer to 7 yards from 10, and I used a much larger target (4x bigger).
After I fired my first double tap, it is a humbling discovery to find that extra hole outside of the 7 ring (the left hole). Being in disbelieve, I fired another double tap at the target and found another extra hole outside of the 7 (this time on the right).
After these 2 double taps, I decided to follow a board member's advice of waiting to fire the 2nd shot until I can see my front sight. Following this advice, I put the rest of the holes into this target.
7 yards, 15 double taps (30 rounds).
By following the board members' suggestions, I saved myself lots of useless double tap drills and the embarrassment of arguing w/ the ISPC scorers at my next club match... "It looks like 1 hole, but 2 bullets when thru. I've been practicing it like that all week"
Thanks guys. Looks like I still have a long way to go in this area.