Registering two accounts from the same client

With Asterisk 1.4 and 1.6 we could have the same client register two accounts on an Asterisk. But with 1.8 this is not working properly. Both seems to be registered, but calls can only be made and received on one of them.

We tracked the problem to be something with how Asterisk remembers peers, and the fact that the client uses same IP and port for two registrations confuses it.

Anybody has a solution, explanation to this problem.

To reproduce, simply take ex a Zoiper and register it twice with two different accounts on an Asterisk server.