On my version of software I cant have a agent login with hardware being assigned and a phone being plugged into it. An x ported number will not login. You could set up a phone with hardware assigned, a phone plugged in and make bridged appearances of it on the phone you want to take calls. Use the login code, auto-in code, and log out code on the bridged appearances and it will work. I am not sure that you dont already have the best solution in place though. It seems that you need calls answered and 2 people sharing the same phone is not the best answer. If you find any mistakes, please consider that they are there for a purpose. And everyone needs a purpose.
Hope this helps, Mikey.