Hi!
The easiest is to have 2 different adapters to target each band because any adapter will work better if it targets 1 band at a time.

What you can do is create several virtual wireless interfaces from wlan0, and run each one of them against a separate band, keep in mind there’s a very HIGH chance of this failing but you can try it anyway, so first do
ifconfig wlan0 down
to disable our interface, then enable monitor mode on it like so
iw wlan0 interface add mon0 type monitor
iw wlan0 interface add mon1 type monitor
This will create 2 new interfaces for you in monitor mode, mon0 and mon1, which you can use in different terminal windows to launch de-auth attacks against different networks.

Greetings!
Diego