Save yourself some aggravation and don't preregister anything - even at the check in counter before each flight. Here's what happened to me recently ... I was on a four segment BA ticket. At each check in I gave different ff numbers: my AA number for the first segment, my BA number for the second number, my IB number for the third segment and my CP number for the fourth. In each case the correct frequent flyer number printed on the boarding pass. Now here's what has happened to date. The first segment has not yet posted. The second segment posted correctly to BA but the third and fourth segments were both posted to my AA account! I have no idea what happened here and I hope I am successful in straightening this out or I'm in a heap of trouble!