You mean two o rings? Pretty ingenious engineering . Then again, that is all depending those o rings are not blemished in any way and won't allow liquid to seep through. Because I've heard of several cases of peoples Subtanks leaking from the top red o ring. Which has even prompted Kanger to CHANGE the o ring in the new Subtank Mini, and make them thicker. Which means obviously enough people experienced leaking to get Kanger to change the type of o ring they use.
I don't know what kind of RTAs you've been using, or how you've been building them. But of all the RTAs I own, the only way to get them to leak is from build errors. If your tank is leaking from the airhole, thats flooding caused by too little cotton in your deck. If you get leaking from any of the seams/threads, then thats a faulty o ring. But either way, all RTAs including the Subtank operate on the same principle, a bunch of o rings are used to seal off any seams or connecting points. There is nothing novel about how the Subtank was engineered, as far as sealing the entire unit is concerned. Its an awesome tank, don't get me wrong, but I'm just saying RTAs have been operating on this principle for a long time now.