2 players with the same connection (can not?)

The problem is as follows: I have a friend who comes to my house when we want to play battlefield 3, but when joining the same game will not let us, to pull the game last in. (is taken for granted that everyone has their own and pc) This issue also came up with the steam platform and solve it by setting the properties of the game launch options, the port that was different from the other machine, the command or argument used was this: (+ clientport 270xx ) where x is the port numbers to open it or want to use. In orinig in gaming properties, it opens a screen similar to steam but in this case it says "Command Line Arguments", my question is, if this option is that you have to use to run the game on the other port and what are the commands to enter.