Multiple adapters with one SSID

[Holger Schurig]

This won't probably increase throughtput.

You simply need n hostapd.conf files with different "interface=" lines
in them. Then start several hostapds manually, each pointing to a
different hostapd.conf.

And if you didn't selected different channels, spaced apart wide
enought from each other, you decreased throughtput.

Finally, from my (limited) observations USB dongles are never great
throughtput-wise.