You can try using a modified condom method for filling the mega dual coil cartos and avoid having dry filler in the middle of the carto which creates the dreaded burn. Put about 15-20 drops of e-liquid into one of the carto condoms, pop off the soft cap of the carto and put aside. Push carto battery thread side down into the condom, wait a few seconds, and remove the condom. You will probably have a few drops left in the condom, and that's ok. Next, put about 15-20 drops of e-liquid into the condom again, and push the carto into the liquid from the open end, wait a few seconds and remove the condom. Now look at the filler in the carto. If it does not look slushy, drip more e-liquid into the open end (avoiding the air hole in the middle) until the filler looks slushy. Gently blow any excess liquid in the air hole out of the carto, wipe the carto - especially the threads - replace the cap onto the open end and vape. I've not had a burnt mega carto yet.
Also, because these cartos blow through a lot of juice, make sure you top off the carto every so often. As long as you keep these cartos moist, they will last a long time.
Good luck, and happy vaping!