I’ve never seen anyone use nitrox only on the second tank. Aldora generally separates nitrox and air divers. Profiles and dive times vary too much if some are using air and others nitrox. The only exception I have seen is for afternoon single-tank dives and night dives, when you have more of a mix of divers. The normal pattern is 32 on the first dive and 36 on the second. I’ll dive 36 if I do an afternoon boat dive or a night boat dive. The tanks we use for shore diving at the Villa are air. That’s the only time I use air.