Parallel batteries are good not only for addded capacity, but they'll actually run a bit hotter (the atty, not the batteries themselves) and the batteries should suffer less wear and tear as well. Atomizers actually demand more current than a single battery is designed to supply, which is why you don't get nearly as many recharges from one as the manufacturers claim. By running in two batteries in parallel, you spread out the amp draw, so each battery only has to "work" half as hard. The atty gets all the amps it wants, and the batteries live longer. If you don't mind the size of a 2-battery mod like this (and are happy with 3.7V), it really is the ideal configuration.