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Question about how power travels through 3-phase wires for BLDC motor

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Apologies as the question is not necessarily EV-related, but this is one of the most knowledgeable forums.

I am tinkering with BLDC motors to potentially use in a series-hybrid model train or go kart or something fun like that. Two identical BLDC motors will basically replace the transmission in a crude manner.

I want to attach one motor to a small engine as a generator and another identical motor will be the traction motor.
The generator will be connected directly to the traction motor and speed will be controlled with the small engine throttle since this does not need to be precise or efficient, it is just for fun.

I also want to use an ebike controller connected to the generator motor to use it as a starter for the small engine, and potentially as 'boost' during low speeds or high loads.

My question is, when I want to disconnect the traction motor from the generator motor, or the controller from the generator motor, is it sufficient to only put a contactor on one of the three phase wires?
Or do the other two wires still stay 'live' while the generator motor is spinning and thus I should put contactors on all three to prevent any drain or potential damage?

I am only familiar with brushed DC where one contactor is sufficient on the positive wire, but I am still a bit clueless when it comes to brushless motors.

Thank you.