What I don't understand frankly, is why you'd be willing to sidemount twin 80s off a boat, but not carry them as doubles. The weight is the same, so whats the primary issue?
OK, 1) The cost of owning doubles, not to mention the possibility of multiple pairs. 2) The weight of hauling around doubles. 3) The problems of filling doubles.
When diving off of a boat that supplies tanks, Al 80's, one more tank is nothing. I dive with a shop that might have 70 AL 80's on board. Walking around the set of doubles laying on the deck, in the way is a pain. When working with students I can dive a single, when escorting/guiding/babysitting certified divers I could go sidemount or single. When not "working" I can do anything. All by carrying along my extra reg which I already own.
In any other situation other then free, easy, AL 80's, doubles might make sense. But if traveling, then I could to rent doubles, maybe rent a large HP steel or use the "free" AL 80's.
tnx don O