When diving side mount doubles on a typical open water "two tank" dive boat do you rent two tanks or four? . . . Just curious what the common practice is in this regard. And just to be clear, I'm talking about recreational open water diving.
If it is a 'two tank' dive, many boats on the Carolina coast mean that you should bring (only) two tanks, although most have no problem with a pony, so a third tank is not entirely out of the question, even if it an AL80 (i.e. full size).
If I am diving SM HP100s (or 120s), I don't bring a third tank. (If I am diving BM double HP 100s, I don't bring a pony / third bottle, either.) I get plenty of bottom time across two dives on those.
If I am diving sidemount AL80s, I will bring a third AL 80 with a standard (right hand) valve, breathe primarily off my right tank during the first dive, and swap that one out between dives. As for the question about rigging, I use a cam band on the bottom, and a bolt snap on a static line loop around the neck, of each tank (i.e. I DO NOT use fixed stage bottle rigging), so changing tanks is very easy, takes about 60 seconds, including the regulator.
If the charter is full, however, I limit myself to two AL80s anyway, simply out of courtesy to the other divers.
The only time I brought 4 tanks on a recreational Carolina coast 'two-tank' dive, I got (appropriately) chewed out by the captain (in a nice way, but the point was clear).