Two 3s packs simultaneously is no problem. The charger has basically two halfs, each capable of up to 5s or for bigger packs you join them (up to one 10s pack). There are two sets of taps and charge jacks. The charger has a 5s Cellpro jack, it also comes with two 5s to 4s cellpro adapters, I then bought some of the 4s cellpro to bare wire cables ($1.99 each) which I used to wire up my TrakPower Packs (I'll take a photo of it when I can.) FMA sells adapter packs on their site for all the common taps so you can plug right in. Prior to this I had the super simple Scorpion charger so this was a big upgrade for me and it's very simple, much easier than I thought.

DM