Right. Ok...
You seem to forget that one 4S pack is essentially two 2S packs wired in series.
As you correctly state for the 4S pack, plugging the BEC input leads into two of the 4 cells in the pack will discharge two of the cells at a higher rate, thus causing inbalance.
Utilising two 2S packs is exactly identical to the above. You plug the BEC input wire into one pack essentially (or as LRP say in their manual, between the two packs), which drains one pack, and not the other. To be able to discharge both at the same rate, and get 7.4V, they'd have to be connected in parallel. To get 14.8V for the drive, they'd have to be connected in series. There is no way to connect two battery packs in series and in parallel at the same time. I read LRP's manual, and there is nothing electronically special about a 'thin black wire'.
It is an SPX with a capacitor stuck on the side - simple as.
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