Is it possible to simultaneously create a single AP on raspberry pi 3 b+ which uses both 2.4 Ghz and 5 Ghz bands so that users with low end devices can connect to the 2.4 Ghz band and users with latest or high end devices can connect to the 5 Ghz band? If so, what is the hostapd for that configuration?
I believe the chip is unable to hold two channels at once. It should be possible if you add another radio, such as a USB wifi dongle, that would support the other band and channel. You would end up with two network devices.
I would love to be proven wrong, of course, as it would be nice to support both bands.