I've got a diagram floating around here of exactly what you're looking for.
http://www.rc-monster.com/forum/show...1&postcount=26
That's the setup I'm using to charge two sets of 2s packs in series.
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