I must've been not quite clear...
there's the scenario:

myWinFormApp
{
myWebservice WS1;

bool myWinFormApp.Auth(userID)
{
 return ws1.auth(userid);
}
}

myWebService
{
myWebService ws2;
[Webmethod...]
public bool Auth(userid)
{
   if (localServer is defined)
       return local Auth(userid)
   else
      return ws2.Auth(userid)        //in the web.config there is a custom
key: "remote server" (made at install)
// and the webservice on that "remote server" is called to authorize the
user if the local one is not defined
}
}

that's the issue: how can I call the webservice on a particular server
(refered to by IP/DNS name)

yeah, after some tinkering, I got it; but instead of addind a reference to
the ws itself  I made a proxy using wsdl and add it to the project ( kind of
reference though...:)) )

thanks for opening my eyes