While two V6s connected by WiFi to your Superhub can work successfully, they almost certainly have to be connected to strong 5GHz WiFi signals.
Your low signal strengths on 2.4GHz suggests that WiFi propagation in your home is not good.
You could try connecting both to 5GHz WiFi. The signal strength will be lower as the reach of 5GHz is lower than 2.4GHz but the connection speed will be higher and may be enough to hold the connection better.
Of course, you should consider using an Ethernet connection, rather than WiFi, via direct Ethernet cable if one of the V6s is close to your Superhub or Ethernet over powerline adapters if not.
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