The only way that cells in the external pack would go over 4.1 volts is if the external pack is mis-balanced, which would be fixed by the BMU over the longer term. The external pack sees the same voltage as the main pack, so one pack wouldn't be overcharged when they're connected to the same charger. It looks like both packs are balanced, and since both still have functioning BMUs, they should stay balanced. Even still, the cells have headroom and can go to 4.2 volts.
With the relatively tiny size of the on-board charger especially across two packs, the two packs won't become mis-balanced, but if they should, they will auto-equalize. This project isn't much different than how Tesla and Nissan battery packs are constructed with multiple cells in parallel (except they are paralleled at the cell level and this is paralleled at the pack level). Since they are the same size, voltage, and chemistry, the two packs' median voltage will result in the SoC of each pack being very close to each other if not the same. Being that the external pack is connected automatically when the main pack is connected, there shouldn't be an instance when the two packs become severely out of balance with each other unless the external contactor fails to engage or the fuse blows. At this point, the main pack must be brought back to the voltage of the external pack before they are re-connected.
Dani, it may be possible to connect an OBDLink to the external pack's CAN lines and have two instances of CaniOn running at the same time. One monitors the car and the original pack and the other watches the external pack.
"Bear" - 2012 Diamond White Pearl ES with QC - 2/21/2013
Solar-powered since 10/10/2013
"Koorz" - 2012 Cool Silver Metallic ES with QC - 1/5/2015
2017 Bolt EV LT in Orange with QC - 7/31/2017
Driving electric since 2-21-2013.