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Rather than give up the safety of redundant air supplies, I recommend you explain to the operator that you have 2 tanks for the safety of redundancy they provide, not to double your bottom time on a single dive. When an operator expresses this concern, I explain to them that I intend to dive with both my tanks for redundancy and balance and I will return from the first dive with 2 tanks over half full (and then use these same 2 tanks for another dive later). This explanation works great for "2 tank" boat trips, as you will only be using 2 tanks for 2 dives, just like everyone else (you'll just have both with you underwater on both dives while the other divers leave one useless on the boat). For a liveaboard, I actually recommend you take 2 full tanks on the first dive of the day, breathe half of each, then swap out one for a full tank for one of them for the second through N-1 dives and breathe it down like you would if you only had 1 tank (while keeping the other at half full, like a 40 cf pony that you don't use - for safety), on the last dive (N) you pick up the other half full tank from dive 1 and breathe the rest of it and the other half full one you still have that was your safety/pony bottle all day (like the 2nd dive of a "2 tank" boat trip). This provides you better safety than single tank diving, but the operator can be assured you won't be doing much longer dives than the other divers.
