Hi Hope. Yes the extra heat is normal, as well as the missing battery life. Thats what the low ohm load does. Espcially when you are trying to run a booster mod(single battery mod that "boosts" voltage above battery level like provari/lavatube/ etcetera). See the ohm load of he carto as a whole is 1.5, now the coils in said carto are 3ohms each, wired together to bring the load down (parallel), this is giving you more heating surface in contact with the liquid allowing more to be heated to vapor at any given time, but the draw on the battery is exponentially higher than a single coil, even of the same resistance.
Since you specifically have a variable voltage device you would be better served to get yourself some single coil cartos or the same (or any other) tank, and run a little higher voltage to obtain your desired results. This will lighten the load on your battery giving you better use time and be capable of providing you with a similar (if not near identical) vaping experience.
Yes dual coils can vaproize much more liquid than a single coil, but they are best used as if they where a single coil resistance (when I use them I typically run them at or above 5.7volts) but this requires a very hefty battery and regulator that is not readily available in a production mod. Lately(a few months) I have had nearly zero use for any dual coil cartos between the Vivi and the Evod tanks, they are too easy to fill and keep clean to diddle with much else. I dont even have any desire to go back to cartos or even to "upgrade" to any of the rebuildable genesis type tanks that are collecting dust in my vape cabinet.
Everyone has therir own sweet spot. Heres to hoping you find yours. Good Luck. Happy Vaping.