Something to note about any BMS that has separate charge and discharge ports (which might be the case with some toolpacks; I haven't opened any modern ones to check):
The body diode of the FETs used to switch charge or discharge off will still allow reverse current to flow regardless of the state of the port, unless there are additional diodes in the current path to prevent this (which is not typical in ebike BMSs, no idea what is typical in toolpacks)
So if you have two packs with separate C & D ports, and each battery is at a different voltage, and you wire the D ports together, the lower voltage pack will be charged thru it's D port even if it has shut that port off for whatever reason.
If it hasn't shut off it's D port, it'll also still charge thru it.
Same for connecting them up by their C ports. The higher voltage pack will discharge thru it's C port into the lower v pack's C port.
Also, if you permanently parallel the D ports, then because of the above neither pack can protect itself during charge from an overcharge if they are simultaneously being charged.
Similarly, if you permanently parallel the C ports, then because of the above neither pack can protect itself during discharge from an overdischarge if they are simultaneously being discharged.
If they have a common C & D port, then the BMS is designed to protect against any of the above scenarios, and they could safely be paralleled...but they should still be made the same voltage (or very close) first, so that there is no inrush current from one to the other that may be higher than the charge FET(s) on the BMS can handle. This can damage the FETs in a way that can short them "on" permanently, so the BMS can no longer control charge/discharge, and thus can't protect the pack anymore. Typically it's a silent failure so you don't even know this has happened, and cell damage can occur eventually if limits are exceeded.
Secondary note: Some toolpacks don't have a BMS in them at all, and rely on the tool or the charger to control pack usage. I have an old Black and Decker hedge trimmer that uses a Li pack that doesnt' have anything other than a temperature sensor in it, and that is wired to separate contacts on the connector. The tool has a board in it that monitors that temperature, and the pack voltage, and shuts off the tool if either exceeds the limits it has. The charger does the same. (oddly the tool can't monitor it's *own* temperature, so the motor can easily overheat and burn out...).