For best results you’d want to use 2 different wireless adapters, one for each band BUT it is actually possible to create 2 virtual interfaces from the same adapter and use one on each band, not very reliable and might not work at all but here’s how to do it :
ifconfig wlan0 down
iw wlan0 interface add mon0 type monitor
iw wlan0 interface add mon1 type monitor
Now you have 2 interfaces i monitor mode, mon0 and mon1, so you can use each one of them in a different aireplay-ng terminal window deauthing the target from 2 different networks.