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.

Elite Dangerous: CMDR JohnnyChemo
MWO: Johnny Keemo

Always be yourself. Unless you can be Batman.
Then always be Batman.