I have had the m6 plus ( hard wire ) for few months now and I just purchase the mini 1 to use in rear. Does the mini 1 have to be hard wire has well for me to use both in the app ?
The mini 1 automatically connects to the wifi from the M6+ after you have them linked. So when you connect to the m6+ wifi you can control both of them in the app while they have power. If your question in whether or not you can interact with the mini 1 through the app while only the M6+ has power, then no. Both will need their own power source.
You do still need a micro sd in both cameras. With the m6+ and mini1 you can link the cameras to use picture in picture and swap between front and back views in real time and during video editing. You can also have the remote button work for both cameras at the same time.