If the single punched cartomizer is working for you then, you should stick with it. Since you said that those are hard to find, a double punched cartomizer shouldn't be a problem. It will probably perform better since wicking is an issue with dual coiled cartomizers. Only possible downside is if they were punched incorrectly. Which may lead to flooding.
You might want to consider punching them yourself. It isn't hard to do and you'll save money in the long run.