They do not provide anything to block the second airflow hole so I'm going to say dual only.
However I'd assume you could fill the second hole with cotton and be fine.
Although, I have never tried that method.
I've tried that method with the boreas and it worked great.
for the supertank, i felt a little let down. airflow felt very restrictive, no way to replace the glass and cleaning it is a nightmare since the class part doesn't come off to get inbetween it and the chamber. I was using the conqueror, supertank, and inde duo last week. felt all of them were pretty weak but if I had to rate them I'd take the conqueror, inde, then supertank. conqueror gave me the best flavor but also dumped a tank on me overnight. supertank had good flavor but not much aiflow, capacity felt small, juice holes felt small and wasn't wicking fast with my 80/20 VG juice and the inde was too big and too much air on the 30mm cover (and the inside glass broke from 1 fall) and the 22mm one was too small and got too hot. If i had to choose one as a daily use I'd take the conqueror and just remember to close the juice control overnight but it also has the same hard to clean set up. I felt all 3 of these were "over designed"