7.4 V will fry most attys, unless they are high-voltage (low resistance), and then it will just fry them later. Its tough to manage 7.4V. I have no experience with the double barrel PVs, but looks like it runs two attys in parallel...it will just drain the batt twice as fast, and they may not power as strongly as one. There is a Silver Bullet DB model (Altsmoke) that uses an 18650 2000 mAh batt, which will still probably last the same as a 1000 mAh with the DB. Its a honkin' batt, so there is likely mega vapor from this
device.
http://altsmoke.com/feature/dbsbtop.html
there may be others, but I'm not aware of them. They would all mosre than likely run similarly, as its not that easy to power two attys at once.
If you want to build a mod, I think it would be a good idea to learn about voltage, ohms, wattage, current, and all those things. In fact its a good idea to learn about them before you start working with a high-voltage mod, like the 7.4V you are considering (2x3.7V in series). The resistance of your atty will be the difference between a good vape and a melted cart or fried atty. The SB DB uses one batt, so is a 3.7V
device, so it won't likely fry the atty like HV does. It might be pleasurable, but it would likely be
way too much vapor for me! But then I'm a lightweight. I'm very happy with my 14500 mod 3.7V with one regular 510 atty.
