This is made by BrianG on this forum
http://www.scriptasylum.com/rc_speed/_lipo.html
Go down a little bit to the part that says "Battery Pack Wiring".
Click "Use 2 series-connected battery packs" a picture should come up of how you'll want to wire it.
Basically join the
positive wire of the first pack to the
positive wire on the ESC.
Join the
negative wire of the first pack to the
positive wire of the second pack.
Join the
negative wire of the second pack to the
negative wire on the ESC.
Some people (including me) make an adapter to join the two packs together, so you can easily go back to using 1 pack. To do that you'll need 1 female plug and two male plugs (assuming your using a plug with both wires in the same plug eg. deans, tamiya, powerpoles etc)
Oh, and I recommend putting that site in favorites. It comes in handy for a lot of things, especially the top speed estimator part.