2 ohm dual coil carto will be fine @ 3.7v, you can even try lower ohm if you want a warmer vape; although I've recently switched to the single coils in all of my tanks. I'm getting moderately more battery life with arguably better vapor production and flavor with these. Dual coils were a nice idea, I just haven't seen the benefit, plus they drain batteries faster.
I recommend you not stack batteries, although I did try it myself for a very short time. This can happen:
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Recommend that you either watch a couple You Tube videos on the proper filling of a carto tank, or read over the threads in the "tanks" section, or better yet do both. Most people's difficulties in using carto tanks is from user error or lack of knowledge.
If your cartomizers are not pre-punched, you will need to decide how to make the hole(s) into the carto to allow the tank's juice get to the heating coils. Some people use a saddle valve tool, most people use a carto punch tool available at most of the online vendors, or you can make a slotted hole with a dremmel tool and cutting wheel if you already have that. There are how-to videos on this topic on You Tube and advice in the tanks forum at this website.
Should you decide you like using a tank, I'd then recommend that you get a tank tool accessory such as the Slap Yo Moma tool from Big Daddy Vapors. The tool makes filling the tank with juice easier and actually prolongs the life of the o-rings that hold your tank together w/o leaking.