Not sure if it was said before. The record will be split WITHOUT asking in some circumstances. It happened to us the other day when we had requested a domestic upgrade with miles and one of the two segments were waitlisted.
Also, when split (no matter when) my companion has always retained my status thought the rest of the flights (even the return).
But, I have had an auto re-booking situation after a misconnect (I was automatically assigned a new flight and my companion was not). I had to ask the gate agent to fix my companions ticket. He snarkely asked why I was asking about somebody else and I told him our records had been split. He then proceeded to help me.