You'll need this:
http://www.modelsport.co.uk/logic-rc...products/32573
And this:
http://www.modelsport.co.uk/etronix-...products/39349
You need to read the instructions which come with the dual pack balance board very carefully, because if you connect the cells/balance taps up the wrong way round it will short out both packs instantly.
Also keep a close eye on things when the packs are almost charged (when the charge rate goes below 0.5A) because in my experience, although the cells in 2S packs are matched with each other, that doesn't necessarily mean they're resistance matched with another 2S even if they're identical so sometimes you can end up with one overcharged and one undercharged pack.
To be honest I gave up doing this about 18 months ago due to this, it's OK if you're really pushed for time but if possible a better solution might be to get another charger.