Install 2 usb voice modem usb huawei

hello everyone, I followed an Italian guide (link below) for installing an usb "voice modem usb " to have a gsm trunk in my switchboard, and all work. The thing is that I would like to have 2 gsm “voice modem” but I don’t know how to menage that, so I would be really grateful if someone can help me to manage 2 usb “voice modem” for the input and output calls. Thank you very much