Separate network with conflict channel

Key limitation is the requirement that the 6 GHz WiFi must be WPA3(-Personal/-SAE), while the 2.4 and 5 GHz band can be operated either in WPA3(-Personal/-SAE), or WPA2(-Personal), or WPA3(-Personal/-SAE)/WPA2(-Personal) mixed mode. Not sure I have the full picture here: Some wireless devices don't like the co-existence of an SSID on different bands, with different security settings (like WPA2/WPA3 mixed) on 2.4 and 5 GHz and the mandatory WPA3 on 6 GHz - these show some odd warnings then at least.

Because of the MK8x system with it's MR80 router and one or more MS80 satellites, and it's design as a wireless Mesh, the 5 GHz and the 2.4 GHz is fixed for the Mesh usage (why ever not the 6 GHz band due to the shorter possible physical reach I've learned, blame me - I'm an managed and SMB network person), these two channels must be the same on the router and the satellites to operate the wireless backhaul - regardless if there is a wired backhaul plugged. This applies to all tri-band Nighthawk Mesh systems, and also to all tri-band and quad-band Orbi Mesh systems. Each radio is set to a defined set of channels, and it can operate only in one band each.

What we can configure in ADVANCED > Advanced Setup > Customize WiFi Network are just additional SSIDs, operated on the same radios. This feature is a result of a more than a decade old complaint that certain 2.4 GHZ-only IoT can't be discovered while the wireless client (mobile phone or tablet with the IoT App) is associated to the (in general preferred) 5 GHz band.

Hope this explains things a little bit better.