I have set up an asterisk server and two x-lite clients. One is on the same network as the asterisk server and the other is on another network. Both networks are behind NAT, but the asterisk network has port forwarding to the asterisk PC. There are also real phone lines and a sip-gate account.
Both x-lites can make and receive external calls through sip-gate / the phone line. Also, both x-lites can take part in a conference using the asterisk PC.
However, calling one x-lite from the other results in one-way audio only.
I have made a diagram of the setup in the following image: