How can I design a simple dual battery e-bike protection circuit?
An easy solution to this problem would be to add a Schoky Diode in series with all 3 battery packs with a reverse voltage rating of at least 70V. You would want to select a Diode with the lowest Forward Voltage since that will determine the amount of power lost. Power Loss = Vf * I. With a larger power loss being one disadvantage, I believe that heat will also be an issue. Depending on how much power is being eaten by the diode, you diode may get too hot for comfort/operation.
A more difficult, but more effective solution would be to use relays as a switch that connects and disconnects the external battery packs. You could probably use back to back MOSFETs as well, but it all depends on how small of a circuit you need/want. Here I have a mock circuit of what you would want to build. The components PN don't need to be identical. Use whatever works best for you.
EDIT: The cathode of D2 should be connected to 12V.

The main battery pack has a voltage divider so that the reference voltage on the Arduino is lower than 5V. For the external battery packs, they use the same voltage divider when comparing the external battery voltage to the main battery voltage. If the external battery packs are lower than the main battery pack, then you could write a program that disables the relay until the voltage on the main battery pack matches the voltage on the external battery pack. A flyback diode is needed on the relays to prevent a voltage spike on the inductors when turning off the relay.
A buck converter is used to step down the main battery voltage, and use it to energize the relay. You may need to design the Buck Converter yourself since I do not know of any COTS Buck Converters that can take an input voltage of 70V.