Using two extensions with ATA Cisco SPA122

I have not programmed the SPA122 in a couple of years, so my memory might be a bit off, but I believe that there are 2 separate SIP setups in the configuration options of this device, one for each phone port.  So you need to register/program each port with RC as if it is 2 separate devices.  RC would see it as 2 separate phone devices with 2 separate SIP registrations to 2 of your phone lines.