You can just tap the BEC Pro off one of the 6s packs..unless you plan on running a 12s cinder block sized battery. The BEC can tap off just one of the battery leads and will only receive 22.2 volts. We have a wiring diagram on the BEC page that covers this.
http://www.castlecreations.com/products/ccbec.html#
It's under CC BEC Resources; CC BEC Multi-pack Installation
It says it's for the regular BEC only, but it will work just fine with the BEC Pro.
Thomas Porfert
Castle Tech Support