Dom is correct. Most aff programs will tag the player to the last referrer because they are the one who converted the player. This makes sense.
You will also get some programs who will tag the player to the first referred affiliate for any account they create at a casino within the group. The best thing to do is read the T&Cs of each program you wish to join to see how they do it, and if there is no info there, contact the affiliate manager.