Questions about RT-AX92U AiMesh AX6100

I consider the main advantage of Wifi 6 to be the double bandwidth, 160 instead of 80MHz. This doubles the possible throughput under Wifi 6 devices.
The device has two transceivers in the 5GHz range, i.e. two 5GHz bands. Both support Wifi 6, with the upper band using four streams and the lower band using two. With devices using four this means a doubling compared to two, in mesh this band is therefore

You only need one. If you have two, you can set up a so-called AiMesh system, which enables denser coverage of the signal between the two devices, but this is completely optional. Personally, I only have one of these devices.