Can't play Co-Op with my brother

Hi,

I've had this issue with my brother for a while, we both
have a copy of CoD WaW each (we're not using the same CD-key)
I can play fine with him on Multiplayer on all servers, but when
we're trying to play on Co-Op server (Zombie, etc) I cannot connect
to the server that he is in.

I don't know what the problem is and I've tried to sort it
for a while now and it's really doing my head in.


Thanks in advance