I had this problem too. I would search but never find the other local Xbox. Turned out I had one copy of Battlefront 2 that was a "platinum hit" release... and another copy that wasn't. I purchased another original copy and it worked correctly without any port changing on my router. I know you have to have (or not have) the same DLC on each XBOX. It looks like you also have to have the same "edition" regardless of any additional content it may or may not contain. Maybe the platinum hit version just don't work with system link. I was doing system link with two Xbox 360s btw.