Hmmm. Interesting. Maybe I need to do more investigation on the doubles.
I finished the wreck diving class, but I still need to do the 4 wreck dives to get my whizzy card for my collection.
Get comfortable diving Jersey with a single tank before you go with the doubles. You can still hit a wreck at 100 feet on a single, but I would grab a pony, and toss the spare air. If your worried about air consumption try a larger tank, or a steel tank.