From what I've learned, dual coil cartomizers have a second coil higher up in the carto. The top will generally dry first, and it's difficult to keep that portion of the carto juiced up. Adding punched holes to the top of the carto doesn't help, as it will allow air in once the tank's fluid level drops below those holes, reducing the wicking effectiveness.
There isn't much point to using a second coil. You'll get a great vape from single coil cartomizers, without sucking down battery life. As you use single coil cartos in a tank, you'll notice that the juice doesn't stay 'topped off', but the polyfill where the coil is located will stay saturated, which is all that matters.
Much like Baditude does, I order my cartos laser prepunched. The ones I order from sweet-vapes.com have two laser punched holes, and those wick quite adequately. Punching the holes yourself can deform the cartomizer, making it difficult to get in and out of tanks.
On that note, get a carto-tool, like the Slap Yo Mama tool. IBTanked.com also includes a tool with every tank purchase, not to mention a 'thingy' (It's essentially a pill bottle with a screw-on lid) for carrying your tanks. It allows you to have a tank with a carto in it and full of juice, without worrying about leaking or spilling. Fiddling with your thingy, or repeatedly screwing the cap on and off, can also push more juice into your carto inside the tank, if it seems to be drying a bit.
EDIT: Ocelot has a video on youtube demonstrating how to hotswap cartomizers in a tank that contains juice using a carto tool.
