Have you swapped monitors to make sure the monitor works when set as the primary before digging further into this problem of dual-displays. Also what is the resolution you are using and does both displays support it. I saw once a while back where an older display couldnt sync up to the high res and so the new display worked and the old display was not detected and couldnt sync. When dropping the resolution down to like 1024 x768 both displays worked, but when going like 1600 x 1024 or something like that only the modern or newer display was able to handle this.
