My problem is that I see no way to open a QNetworkSession from one of those configurations choosing the hardware interface.
Is not something you (or your sysadmin) did already when configuring the networking devices for your system?
Not that I have that much experience with QNetworkSession, but it looks that behavior to be fine, given that you are setting one WLAN interface to one SSID at OS level. Thinking of wpa_supplicant on Linux for instance:
and then command line:wpa_supplicant -iwlan0 -c/etc/wpa_supplicant.conf -d
Because at the end it'll be a question of OS routing, won't it?