Yes, you cannot just throw a second one in there. The charger itself has to be programmed for slave operation (unless it was already), and then the car has to be told to use it, otherwise it will not. Then, you also have to flash the firmware.
In addition, the installation involves breaking into the glycol coolant lines, so it's messy and requires a coolant bleed afterwards.
I've done it and unless you REALLY have a specific need for higher power, it's not really worth it.
If you are really set on one, I can assist with the installation, but I can say from experience; not worth it as you have to carry it around all the time from then on, while most of the time it's not really used/needed.
Somewhat OT, but I'm working on a project that's basically a "mini" supercharger. It will use 3 charger modules and be capable of up to 36kW. I will install this at my shop where we have 3-phase power and then on the rare instances where a quick top-off is needed, we can use that rather than carrying around the extra weight of the 2nd charger in our cars.