To fill a dual coil, or any other carto:
1st: DON'T pre-fill it.
2nd: DON'T try to fill it through it's normal fill hole.
Install the carto into your empty tank then press the threaded end of the carto back into the inside of the tank and move it over to one side out of the way, just enough to drip into the tank without dripping onto the threads. Keep the threaded end pointed UP. If you use a syringe, push very slowly so as not to pressurize the carto.
Stop filling before the juice gets to the threads. then just push the threaded end back into place.
It is normal for some juice to get in the air hole. sling or blow it out and your set.
With that being said,,, I just slam it in through the fill screw and sling the excess out.
(but I'd guess that you don't have a fill screw)
you can even push one carto out with the new one and not loose more than 1 drop.
Also don't blow back through the drip tip unless you use CE2s. It will pressurize the tank and get all gurgley on you. If you need to clear the air tube/passage blow through the threaded end. But a good 'ol sling (or 3) works better.
It is best not to fill a carto tank all the way full. leave some room for the juice to expand some as it heats up from the coil firing. Or else it will pressurize the tank and get all gurgley on you too.