I've never seen this done, but I'd imagine it creates a loop. When a call is connected to a line with call forwarding, it does first connect to that line before forwarding to the next. I'm pretty sure it'd make both phones ring endlessly before it was disconnected. the loop may break if someone on either one of the lines attempts to pick their phone up, but I'm not sure. I think the delay between connection and forwarding, though, may be enough to cause the two from overloading eachother or causing an error. Is anyone able to test this?