I think that may be the case. Here is a recent experience. I was with my wife (she is SE) on the same PNR. Checked with the WC agent and she said there was 1 FC open. I'll not be able to get it unless I split our reservation, then I'll be #1 and will get seat. At the gate, sure enough the GA called and I got the seat. As it was a flight from the west coast, it would be difficult for me to imagine there are no GE or SE who would have gotten the seat had I not split the PNR. It would be fine if we decided not to split and seat in Y. Some other person would have been happy.