We're talking about dives within recreational limits >140ft and due to the fact that most of the dives out there are 95-100ft you inadvertantly end up with 7-9minutes of time to surface after it is all said and done. So 4-6 minutes of 'decompression'.
Diving my 130's in the NC profiles I mention above, I tend to get back on the boat with 1100-1200psi, so clearly I carrying too much air.
Ideally I'd be able to 2x up my 100's and get two NC dives on them and a bit of reserve, but I think it would be cutting it close on that 2nd dive.
I was taught the "alligator eats the big one", so greater than 140 = >140 and less than 140 = < 140.
Diving double 100's in NC is exactly what I do. My real world working SAC runs around .6 to .65 and in my experience I will run out of no deco time before I hit turn pressure on a dive in NC. It works out well and I will normally finish the first dive to 100-120' with 2000-2100 psi in the tank after a dive to the NDL or to somewhere past it with around a 3-5 minute stop. The second dive works equally well - longer but shallower so again at the NDL or with a short deco stop I'll come back aboard with 700 psi or so. 700 psi in a set of HP 100's is still 40 cu ft, so it is a healthy reserve for a recreational dive or short deco dive and is still basically "thirds" for the second dive as you are using about 1400 psi of gas per dive.
If your SAC is a bit higher or your dives bit deeper or longer you can take along a 40 as a stage or even go with an AL80. I recently started using an AL 72 for a buddy bottle and stage bottle for cave diving and it is ideal as it is a bit skinnier and is near neutral when near empty so it does not get as floaty as an AL 80. It would also be a great way to get more gas on a pair of deeper or longer NC dives.
Two sets of doubles take up a lot of space and that ma not work well on a "recreational" NC dive boat. But no one seems to object to a set of doubles and a stage, so it works well to use it on dive one, to keep more gas in the doubles for dive two. If you use a stage in that manner, just take it on the first dive and breathe it down to 200 psi, then switch to back gas to complete the dive. With an AL 80 and 200 psi left over, that would give you basically 70 cu ft. of useable gas on top of the 200 in the double 100's. Just figure your turn and switch pressures for dive number 1 to ensure you end the dive with enough gas to meet your requirements with a 1/3rd reserve on dive two.
Personally, I think it makes mores sense to carry an AL 72 or AL 80 stage on those rare days when I go deeper and need more gas, than it does to carry double 130's all the time. An HP 100 is 33 pounds while an HP 130 is 43 pounds so with the extra gas included, a set of HP 130's will weigh about 24 more pounds than a set of double HP 100's. That can be significant as with double 100's, steel back plate, can light, wing, and a pair of lift bags, a total weight around 120 pounds is realistic and another 24 pounds makes getting up the ladder in 6' seas much harder.
An AL 72 set up as a stage weighs about 32-33 pounds so you end up carrying 8-9 more pounds total, but you can send the stage up separately and not hav to cary it up the ladder and you only have to bring it and carry it on the days you do the deeper and longer dives, not the other 90% that can be done with just a set of HP 100's.
It's a lot like buying a plane. Many pilots make the mistake of buying a 4 or 6 seat aircraft when they only need 1-2 seats 90% of the time. They are almost always better off spending half the money purchasing and flying 2 seat aircraft and then renting a larger plane on those rare occasions when they actually need 4-6 seats. Same thing goes with a set of doubles - buying overly large tanks for the rare "worst case" dive just means you get to haul a lot of extra gas and weight around to no real purpose or advantage on 90% of your dives.
