When you say that you can connect to the master server, I presume you mean the Gamespy Master Server. The gamespy master server communicates with your routers IP and not your desktop or laptops internal Ip adress. So what happens is that if you have both connected, both your laptop and desktop are using the same external IP(router) and as such the master server does not see the difference between your laptop and dekstop and the router chooses either one of those to communicate with.
Unfortunately I'm not techy enough to tell you how you should workaround this. I have never attempted such a set up. But I know its possible allright. What you can do, is explain the situation on the english cdv forum, perhaps Stoneveld or VonSmeed, the administrators there might know how to handle this.
Sorry that I can't help you any further with this, unless someone else knows?