You can use this device as a cutoff:
http://66.241.195.91/lithium/lithium.asp?path1=lithium
It connects to the full voltage of both batteries, and goes inbetween the receiver and the controller's receiver wire.
SH Z-Car, Custom Crawler, 8s Savage,
12s XTM XLB 1/7 buggy,
4wd 4-link rear/IFS Pro4 truck,
Custom Hyper 10 Short Course,
Belt-Drive Mammoth ST 1/8 truggy, 4s 17.5 MM Pro HPI Blitz