That makes sense from a more is better philosophy, but not everyone adheres to that philosophy in all situations. Volume issues aside, twin 119's, deco bottles, lift bags, reels, lights, etc it all adds up and it all has an impact on bouyancy and it all has to come up the ladder at the end of the dive so more may not alwyas be better.jacksatlanta:
It also helps to know a SAC rate/RMV before making decisions on how much will be enough for deep technical diving situations. In general though, with a reasonable SAC, smaller tanks will still give you a useful amount of gas for tech purposes. Even doubled steel 72's or AL80's are adequate for 20-25 minute bottom times in the 150-160 foot range with a 1/3 rd reserve.
Deco gas also comes into play in the overall planning with consideration needed both for the deco required as well as contingencies for losing one of your deco gasses. If you are using one deco gas, you need enough reserve back gas to do the much longer deco required on your backgas reserve and this rather than the 2/3rds available on the bottom can become the limiting factor on deeper dives.
If you are using two deco gases, which is much more likely and efficient on dives below 160 ft, the lost gas contingenecy plans will include either of the remaining deco gases. Depending on the volume and percentages of deco gasses carried the backgas reserve may still come into play but the volume needed will usually be much smaller than on a single deco gas dive.
In short, I would in no way say that double 100 ft tanks are not usable on dives below 160 ft. - there is a lot more to it than just back gas volume.
