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Yes, but you have to continuously charge / monitor one or both packs. If you're tapping power off of your motor batteries, then your "receiver" pack is always charged. The backup 2S-350 is only discharged while you are checking redundancy of the system.
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If you're using the F3A Unlimited adapter, make sure you are using a regulator to feed power into your radio. The raw voltage ripples off of the two motor cells while you're running the motor due to ESC switching. The regulator clips off the voltage below the ripple level and feeds smooth power to your radio.