Charge the 13Kwh TWIZY with an external charger

I drive a 13kwh Renault TWIZY.
I drilled a hole in the original drive battery [6kwh], took out the positive and negative wires, and brought them up to the top,
connecting it in parallel with a 7kwh Nissan Leaf lithium battery.
So, I have a question.
What happens if I connect them in parallel with an aftermarket DC charger and charge them? Will anything go wrong when I turn the key on after charging is finished? Please let me know.

Has the leaf- module an own bms? The single cell voltage MUST be monitored by a bms so that the charging will stopped when a cell is reaching the recommended highest voltage (e.g. 4.2 Volts). The same is necessary for the „old“ twizy-battery. If you connected your new cabling inside the battery without a bms-controlled relay or switch, the charge won‘t stop charching when a cell is reaching its recommended maximum voltage. One effect is shortening the cell life in the long term due to degradation or shortening the cell- and twizy-life very shorttermed by spontaneous inflammation caused by overcoltage.
I recommend some reading in lithium-battery-technology and the basics of bms

Thank you. I have a BMS active balancer. What happens if I charge it with an external charger? Will it detect an abnormality when I turn the key to ON? Will the car stop working? I would like to know.

It's no problem to charge the battery with an external charger.

When you turn on the Twizy again, after you charged the battery with an external charger, it will recognize the new SOC, but it doesn't care about the fact, that this wasn't done by the internal Twizy charger!
Your Twizy will act as usual, as long as you don't damage the battery by the external charge.

It is a bad idea. Even paralelling two batteries without constant individual monitoring and individual safety contactors is a recipe for burning down the twizy and everything in its vicinity.

The question about the functionality: every charge bypassing the original current sensor or while the bms is inactive will mangle the coulomb counters in the twizys BMS, which will lead to errors.