You can only use one at a time, but you can switch them out. So if the tank I'm wearing runs low I can equip the tank in my inventory and I've got more. Like I mentioned, even though I fully tank up each before going under water, when I switch to the second tank it isn't full. I imagine the act of switching tanks causes some loss, so that could be the reason why.
I've only got one part of the SeaMoth scanned so far, so I'm still doing things the hard way.
Throughout history, poverty is the normal condition of man. Advances which permit this norm to be exceeded — here and there, now and then — are the work of an extremely small minority, frequently despised, often condemned, and almost always opposed by all right-thinking people. Whenever this tiny minority is kept from creating, or (as sometimes happens) is driven out of a society, the people then slip back into abject poverty. This is known as "bad luck.” -Robert Heinlein
