Cannot join from the same IP(?)

I bought Monday Night Combat and sent a guest pass to my girlfriend who is interested in buying it.
As we live in the same flat we share the same IP adress.
Everyone of us has their own steam account.

But it seems like we cannot play at the same time.

When one of us entered a server the other one couldn't join. The message "connection to the host hast been lost" appeared.

When trying to host an own game the same error appeared.

The telnet fix posted by Morello also saved this problem.

http://forums.tripwireinteractive.com/s ... ostcount=4