Cell packs on B/L-ESC (9920)

That picture is correct.

A 2 cell pack may be a bit of a nuisance. You'd have to charge it separately (or connect just as you would on your vehicle and charge it w/ your packs), then you need to secure it to your ride separately, finally you're exposing yourself to cell imbalance if you don't use them every time. Of course If you have a conditioner that point is more or less moot.

Sounds like a lot of trouble. I personally would solder 'em up and 'forged about it.'